While you control a monster, your opponent can't Normal/Special Summon, nor return a monster to the field, if doing so would result in them controlling more monsters than you do.
● Your opponent can Tribute Summon, but not if they would control more monsters after the Tribute Summon
EG, if they already controlled more than you, they can't Tribute Summon.
EG, they can't utilize "Soul Exchange" if the Tribute Summon would cause them to have more monsters than you.
● If a monster would return to the field, causing a player to control too many monsters, it's sent to the GY.
This effect never prevents:
● Flip Summons
● "Gemini Summoning" an already face-up Gemini Monster.
● "Take control" effects, EG "Mind Control".