In terms of the count in apartment units. Did/could and how your machine handled it? How's the whole experience storywise? How many sims can you host there resided? How many families (with offspring/roommates)? The Number of Levels? The Style of the apartment/district/block?

Unfortunately, no pictures to show, but my daughter once decided to build a cheap industrial-looking apartment building.
It had 20 units I think, on 3 floors....Units were very small, and crappy. She also made sure that bedrooms were adjacent, so sims trying to sleep would be waking up by noisy neighbors.
basically, a very unpleasant place to live, but sometimes..you just don't have the choice.
It played really well despite of the number of sims on the lot. All single sims of course, because this is how to game assigns apartment units.
it was so ugly, she did quite good! lol

Biggest one I've built is the luxury apartment complex I have uploaded here on MTS. It's three stories with apartments, and a roof top communal area. There are 12 apartments total, floor 1 and 2 each have 4 medium size apartments, third floor has two big ones and two smaller ones. Every apartment has a balcony, and all except the two smaller ones have two bedrooms. There's no particular style, it's a mish-mash of different apartment buildings I've come across in real life. I wanted something higher-end with plenty of room, and a higher-end feel, and I'm quite pleased with it. It plays fine for me, but it'll be rough on lower end machines. I rarely have more than one playable living in any apartment complex, through three different families have lived there at some point or another. I really enjoyed playing in it, with all three of them. Best of all was Elna Sernman, who moved there straight out of college, and through sheer luck several of her best townie buddies from college moved in as neighbors (used mod to make them graduate with her).

Downloaded lots of the bigger kind my two favorites are both by Karen Lorraine, Ruberry Apartments and Queensway Apartments.
- Ruberry is only a 1x3 lot, but it's tall enough to qualify as big in my book. I think it has 6 apartments, some two-story, some only one. Has a communal gym, computer room, pool room and roof-top terrace. I use it as an apartment complex on campus, since the size of apartments and the type of communal rooms feels appropriate for campus. But I also have a copy in my main hood since I like it as starting apartment for richer Sims
- Queensway is 3x3 lot, with two stories and 6 apartments. Again some are single-floor, some have two floors. The apartments themselves are all cramped, but still feels like a big complex to me. It's also a great starting place for my Sims. I've modified it and added a basketball court and a playground, which my Sims really enjoy along with the pool it came with. I've also converted the two top floors of the entrance-structure to a laundry room and a tiny gym.

Biggest apartment blocks Ive built have been 3 or 4 stories high with about 8-12 apartments. I had three of the apartments occupied by my sims. Probably enough not to cause issues. When any kids start going to school I tend to get families moving out and buying a house. One apartment block was a run-down thing with graffiti'd walls (pic in pics section under apartments I think). The other two were quite luxurious, one with a pool. I downloaded a tall one but haven't been brave enough to use it yet.

I build one similar to Rosebine's daughter and townies kept coming and coming until the place went to a crawl. It might have been a glitch.

Apart from that my largest apart is the one I just uploaded. http://modthesims.info/download.php?t=609473 There are only 5 rentable units though.

When I first got all the EPs, I had a bunch of college graduates, and I built a 12-unit building (4 apartments each on 3 floors) in Black Mesa with small, cheap apartments and communal facilities until they could afford a proper place. Was a bit disappointed when I could only get four families in. I'll have to dig out my old backup and see if the building's still up.

Biggest I've done is my Inequality District skyscraper, both in terms of the number of rentable units (22) and the size/cost of the lot (it's just ONE wall segment shy of the limit, and more than $1.5M total cost). Can't say I've really played it beyond just the testing aspect though.

My barracks has eight rooms, but they're not really apartments. Four of the rooms are just a 3x2 space with hall bath, and the other four are 3x3 with private bath. I did a slightly bigger one for penniless civilians: three single rooms, two 4x4 rooms with bathroom, and one slightly larger room with a bathroom and 2x4 'baby room'.

I usually limit all my buildings to 4 units. A. for performance reasons and not generating a bunch of the socialite apartment dwellers, and B. Because I hate not being able to use the units.

Most ever is 8. 5X2 lot that's a city block with townhomes and apartments over stores. Different styles for each building, 4 stories max. 4 families, I think 13 playables. Playing the household in the largest unit was slow going.

100 to test the mechanics. It didnt last long
Normaly play wise was 10 units till i was informed that only 4 will ever be useable by playables

Nope. Might be in the lot bin instead. It was pretty awful though.

The biggest apartment lot I've built was an ultra-modern 12-storey tower with 4 units. There were two 2-storey apartments, one 3-storey apartment and a 4-storey penthouse, as well as the lobby in the ground floor. Prices ranged from 3,5k to 7k a week. It was built in the final days of my old computer, though, so I never got a chance to actually play it before it broke. It would probably lag a bit, as it was already lagging when I was building it. Also, there was one specific grid tile that made my game crash if I tried to build a pool there.

In terms of units, I have built bigger lots, with 7/8, but each unit is smaller so I never had any problems with lag, even with the larger amount of Sims.

Quick off topic question about the outdoor alleyway 'apartment': did you use any cheats to build it or will the game allow you to make an apartment with no roof? And did I understand correctly that you used those 'boundary between two floor types' fences as the boundary of the apartment? I love your lot and want to make something similar but on a much smaller scale.

i never build or play in apartments but itlandmsims built an apartment with about 40 units i think in his prosperity hood micropolis.

@Enoby - No cheats, the game doesn't care if there's a roof or not (though Sims will, since they'll get rained on, but that was part of the point here), and yeah, it tricks the game into seeing an apartment as enclosed while still allowing Sims to enter via a path other than the front door. Take a look at this thread to get an idea of the way the game will handle that kind of setup.

The physically largest apartment I've played is actually an adaption I made of @Zarathustra 's Penthouse Tower. I rebuilt it because I wanted to flip the building around so the units faced the ocean. But it's only 5 units, so not the biggest by this thread's standards.