I haven't played the games in ages... now that I'm getting into them again, I'm trying to remember... was there a limit on how many skills each character can learn?
Or can a single character learn every skill in the game (at least all trainable ones)?

There's no limit. All characters can learn all skills (except thievery which you need to have at the start).

Good to know. Thanks.

Any character can learn all skills with the exception of thievery.

In the "secret" city of Shangri-La (under the Volcano Cave on the Clouds side) a character can buy all skills (excluding thievery) for 100,000 gold.

Your post is about World of Xeen, but in Swords of Xeen, you can even buy the Thievery skill in the second town. The engine supports it, they just didn't make it possible in World of Xeen.

I thought you get thievery when you buy all skills in Shangri La or do I confuse this with MM3?

You confuse this with MM3, in WoX there's no way to get the thievery skill except creating a robber or ninja.