↪ The more bases you have on the map the higher the group limit. 1 Base -> Group limit 3,1 2 Bases -> Group limit 4,5 3 Bases -> Group limit 5,5 4 Bases -> Group limit 6,3 5 Bases -> Group limit 7,1 6 Bases -> Group limit 7,7 etc. group limit on map formula = ((bases_on_map / 10) ^ 0.5) * 10 ↪ On the home map you get 2 more groups limits (1 Base -> Group limit 5,1) ↪ Each group limit extension building increases the group limit by 1 on the map. The more one has of these buildings on the map, the more expensive they become. On the home map they are cheaper.
How many units fit into a group?
↪ It depends on the type of units. Small or light units can be very many; big units only few, or even one unit per group. In the build menu is an entry "Group", which shows how many of this unit type can fit into a group.