The only limitations are your ability to carry the load and in a manner that you don't violate RCW9.41.270.
If you are carrying so many that you run out of room on your belt, in your pockets, on your ankles, etc. and you have to resort to carrying a pistol in one of your hands, THAT might be an improper display.
It would seem that in normal circumstances more 2 than might be a little much. I did have an FBI agent once tell me that many of their people carried "two up and one down". One in a shoulder holster, one on the belt, and one on the ankle.
As for me, it is only two. Usually my Sig and a KT P3-AT in a pocket.