No specific number can be given, because it depends on the size of your army and the size of the defense, but in short, the more, the better.
Try some simulations with your army. Put up a defense with a level 20 wall that will destroy 95% of your hammer. Now double the number of rams, and add a few hundred more for good measure. Do more of your troops survive? Then you need more rams. A lot more.
As a general guideline for this stage of the game, there are a lot of 50-100k hammers running around hitting 50-400k defenses. These hammers should ideally have 1000 rams at the very minimum. When I see a 30k+ hammer with only 200 rams I think "Gee, what a terrible waste of a hammer. A 20k hammer with 500 rams would do almost as well..."