The answer to this question depends on how the 5804's are programmed. Each button has a loop that is considered a "Zone" so if you have 4 fobs and you are going to program all 4 buttons on each one, you would effectively use 16 zones These are called "Key Zones".
You have 16 "Key zones" allocated on the Vista 20P/21IP series panels. These are zones 49-64. You could add more than that but you would be using up normal wireless zones.
The 5804 Installation Instructions are attached for your convenience.