Two in the putter side pocket. They go in and out better if you place the flight plate to the bag instead of to the outside.
I can generally get 18 in my bag (+2 putters on outside). I have, at times, gotten as many as 20 in the bag itself if it was distant or fairway driver dominant. 18 can be tight if mid heavy or even less if I carry more than 2 putters.
My typical set up is 18 + 2