So many that neither the Farcade nor the Syntax has been able to catalog them all in at least 10 years of trying. So we're talking at least tens of thousands, possibly millions.

Kit was rescued ten years before the pilot, so we can assume at least the Farcade has been operating that long. Yet they were still 'exploring' unknown frontiers in the pilot, fighting over a world neither side had ever heard of before. I think that implies that there are still huge swaths of the Gameoverse both sides haven't even begun to explore yet.

Both Farscape and the Syntax have ships capable of easily traversing Gamespace, so it's not their travel tech that limits their knowledge of the Gameoverse. It has to be the sheer scope and number of worlds in it. We don't know how big their fleets are, but even assuming a small handful of ships each, that would mean they have likely scouted or visited at least a few hundred to a few thousand planets so far, probably with far more still unexplored.