There are no particular guidelines, however here are some suggestions.
Sunday would be the NORM for celebrating First Communions. Yes, there can be a large number of children and when family and friends are added, this can create a large number of people in addition to the normal Sunday congregation. However, this is a communal celebration.
Consider then having a special Mass on Sunday for celebrating first communion.
Or consider splitting up the First communions and doing them over several weeks.
Use your normal Sunday Lectors for the readings. This helps in a couple of ways. When using the kids to read there is always the tough choice of who to select...and plus usually the kids are not good lectors. Rather, use your normal lectors eliminates this all. They can make a beautiful celebration for the children and the families.