WS was originally envisioned to include an upwards of 30 countries believe it or not, but after Disney could not find enough sponsorship commitments from companies within these countries, that number dropped to eight or nine. But, of course, eight or nine pavilions around the lagoon look kinda scarce, so suddenly what was supposed to be just pavilions turned into full mini-lands where the architecture and design of the nation extended beyond just one pavilion into multiple buildings and areas. Because of that, the capacity is about 20 or so though that can vary based on the size of the countries.
Unfortunately, at this rate, I'm doubtful we'll ever see (within my lifetime) even a couple more countries. If any change is to be made to WS in the foreseeable future, it would be the addition of DVC villas to the upper floors of some pavilions (that's not a joke! the project is/was tentatively titled "The Villas at Epcot"). That said, the plan for future DVC development has recently changed dramatically, so even that may not be a possibility.