They do not have a limit on the number of transports they will make or send. What they have is a level and situational problem. IOW play at a higher level against a stronger nations and you will see more units coming.
That said, they are still poor at invading, even at Sid. I have seen many stacks of ships coming for me on Sid maps. I posted a turn where the Montie sent 32 galleys at me.
I still do not see many fleets with more than 4 galleons, but what I do see is a long line of boats coming one after another.
Now, if you are playing a level like Monarch and are crushing them, do not expect much to be tossed your way.