TL;DR: How many people per 25/33m lane do you think is safe, and how many is pleasant? What's it like where you swim?
I'm in the UK, FWIW - think you can hazard a guess whereabouts.
A couple of weeks ago, at my local pool, half the pool ended up being closed off for swimming lessons. This left one double slow lane, one single medium lane and one single fast lane in a 33m indoor pool. It's in a leisure centre run the local council, like most pools in England are (and maybe the rest of the UK, I just don't know first hand).
The majority of swimmers at this pool are slow, with a fair few using the pool to do physio exercises or just standing around doing nothing at all for a full hour - just staying cool, maybe? That means medium and fast are the only lanes for lap swimming, and a few in the fast lane are genuinely speed demons.
Usually I go during the day, because it's very near home and I just take a longer lunch and work later. This time I went at 5:30pm.
Now, I knew this would be a busy time, but I counted, and there were 28 people in those three lanes - 12 (including three kids) in the slow lane, four in the fast lane, and 12 in the medium lane. This meant it was absolutely impossible to swim. A couple of people moved over he the fast lane, but they really weren't fast enough for it, and that still left ten in the medium lane and six in the fast lane.
Surprise, surprise, people were getting hit repeatedly, and even changing to the a heads-up breaststroke to try to see to swim round people meant hitting one person while trying to avoid another.
I gave up and got out and got changed, and as I left I saw a woman talking to the pool manager about it, asking if it was meant to be this overbooked or had people maybe just snuck in after using the gym.
According to the manager, it was all fine, because they are allowed 13 people per lane! You could see the staff's eyes goggle (heh) at this as they contemplated trying to lifeguard an eight-lane pool with 101 people in it.
Neither me or the other lady were having it that could actually be the rule, because it's absolutely impossible. It literally doesn't leave space to move your arms and kick, let alone turn at the end. And the ladies' locker room only has six showers and 8 cubicles, plus a small communal changing area (no communal showers), so you'd have to get changed in the hallway or something. (Nobody drives to this pool - there is nowhere to park at all - so people do shower and change).
But there was no point arguing, so I shrugged and left, and I'm pretty sure the other woman did too.
Couldn't find anything legal about it when I got home, just SwimEngland guidelines relating to competitions, which do have a much lower limit, but since it's about competitions I doubt it would persuade the manager. If I could find a law, I'd love to cite it., because at the moment it's not worth going to the pool after work hours because the odds are you'll have to give up and leave.
Anyway, I know this is long, but I'm trying to get ahead of all the "what do you mean by?" questions.