You could only use so many.
If a shuttle was attached, then you could only use so many boosters. The wings of the orbiter would interfer with successful separation of the extra boosters. If however you have a launcher like Energia, then you could completely surround the core stage with boosters like was planned for Vulcan.
I'd like to see a 10 meter wide version of Energia/Vulcan fitted with 5 RD-0120 main engines and surrounded by 10 Zenit boosters.
