About the only things that have been able to rack up impressive numbers of games released that started as games for home computers are Zork and the Old School RPG brands.
There have been 8 Zorks (not including the Enchanter series - Enchanter is arguably Zork 4),
9 Wizardries (Nemesis, which is arguably not a Wizardry game, pushes it up by one),
11 Might and Magic games (including two action games and three Heroes of M&M, not including the add-ons, stand-alone Shadow of Death, or the 4(?) Heroes Chronicles games),
13 Ultimas (including Ultima 7.5, 2 World of Ultimas and UO, not including the consoles, of which I know there were at least a couple, and not including the two add-ons)
I have no clue how many Gold Box AD&D games SSI put out back in the day, but there were a lot (and no add-ons).
I'm a little hesitant to include add-ons in those totals despite the fact that they were apparently used in the initial post. Even without the add-ons, WC racks up an impressive 11 games (including SOP, both Privateers, and SWC, but not including console ports of the PC games).
That puts WC ahead of Zork and Wizardry. With add-ons, I think it still loses out to Ultima (Both Ultima 7 games had an add-on, and I think there have been four for UO, although I could be wrong - total of 19. Plus, there are the console games.).
Might and Magic gets boosted to 18 with add-ons.
WC has a total of 17 with add-ons, not including speech packs, KS, or 'Deluxe Editions'.
To the best of my knowledge, none of these other games have made it onto the big screen, although there have been book series (Might and Magic, Zork, Ultima - IF you count the books for the now discontinued UO2).
Incidentally, with add-ons, Command & Conquer jumps from 4 games to 10.
Did I miss anything?
Is there another big series that got its start on the PC?