There's no real physical limit right now because we have a LOT of mmWave frequency up in the 26, 28, 39, and 41GHz bands. The problem is that the higher the frequency the less propagation you get, the more often you need a base station, the more the network will cost.
The problem on the low band is that you can eventually you get to a point where the cell is overloaded and you're at capacity but still you can throw in another cell and/or split the sectors up. Which, again, costs money.
The problem is essentially economic, not technical.
What is the actual physical limit for the network bandwidth for the available frequency bands? Have we reached it in congested cities?