I read somewhere (here?? probably...) that a limited number of girls is allowed in each session. 60 or 70 I think, that's what I saw (also our 1st comp season), so there were about 6-8 teams in each session (maybe more). 3 best scores on each apparatus count, so you need at least 3 gymnasts (same level) to have a team.Of course, the more gymnasts, the better chance to place as a team, as you can leave all the weaker scores out.
The gymnasts are split into age groups for awards purposes, and they compete against each other within their session. Sometimes (big meets) there are more sessions for certain level.
Numbers from Mymeetscores (one of 'our' meets)
64 gymnasts in 'our' session
8 teams
one of these teams had only 3 gymnasts, while the largest team had 17
At this particular meet, there were 184 L3 gymnasts, so they were split into 3 sessions.
Hope it helps.