it all depends on the organisation of the race.
some races, i.e. stratford,one starts every 15 seconds. you do 4 lengths in the first lane, clockwise, dip under the lane rope then 4 in the next lane and the next, 12 lenthgs of 33 m pool, you then get out at the other corner.
others may have you staying in the one lane doing your 16 lenghts, standard 25m pool, 400m swim. they either start 4 or 6 together depending on size of pool, number of lanes. then maybe start the next wave of 4 or 6 a few minutes later, maybe 3 mins, this way you shouldn't get more than 3 or 4 in a lane,
the other way is to start someone in lane 1, 1 min later someone goews in lane 2, then 3 etc.. up to lane 6, then back to lane 1, i.e you have six mins before the next one gets in your lane, the interval may of course be 30 secs
generally you have plenty of room and i've not seen more than 4 (ish) swimmers in one lane.
is that clear? sorry for rambling.