I guess they'd have to make the center rocket stronger to handle the added force of more rockets. Also, there may be diminishing returns when it comes to addin

I guess they'd have to make the center rocket stronger to handle the added force of more rockets. Also, there may be diminishing returns when it comes to adding more boosters, and at some point it makes more sense to just redesign the rocket to be bigger (hence the BFR, which is the planned successor to Falcon Heavy).