How or where do you find the total # of planets in-game? Meaning the total planets of the game, not player's planets.

It tells you in the box that pops up when you hit "Scan for New Planets."

Ah, thanks.

I figured it was players and planet saturation; but I wonder how inactive players are dealt with. Over time I guess good planets would be taken, but poor planets would still count for inactive players which means over time it might affect the "effective saturation" by active players. In other words, there seem to be more owned planets by active players than there actually are. Although, this should actually increase the # of planets generated. Does that sound correct ?

Indeed it does; so inactive players really don't hurt us or anything, especially because we can just take any good planets they had.