Can someone inform me how many abilities one character can develop? Can I become fully craft able with all abilities on one character? Is there a reason for alts other then bank space?
1) I don't think there's a cap, but I've heard conflicting reports between 50 skill lines and infinite.
2) Sure. You can use your skill points on whatever you like.
3) If you'd like to play a different class, sure. But no, not really. The bank is also shared across your entire account. So I don't really think they considered "alts" when they made ESO. Which is fine, except if you are Templar who wants to do anything but be the token healer in truly competitive Trials...........that's essentially the only exclusion that exists right now.
You can master them all on one character but there are reasons to have alts.
1) You can only research 3 items at a time for each profession. Clothing and Blacksmithing can be a problem because you'd need to pick between medium/light armor or heavy armor/weapons in order to get your research complete in a reasonable time.
2) Skillpoints. You need a ton of skillpoints to master every profession with every passive. This severely cuts into the points you could use for class, weapon, and guild skills for adventuring. Creating alts lets you spread the points out a little better.
3) Storage space is limited. Even the bag space from alts is huge. You'd have a tough time keeping all your mats on one character.