If you have all 4 boos, it'll summon 4 boos (no duplicate boos), believe you need to have them all in belt to summon
I wish that the devs would make the belt work with the pumpkins too. I'm not a huge fan of the boo animations unless I'm on a Scooby Doo character. ZOINKS!!!! I will spawn 2 or 3 if that is all you have.
Actually wrong answers so far tho. I seen player, different people, spawn 16 boos from Their boobelt I think this a over exploit and a glitch.