I think it's tied to your thievery skill, though I believe certain items aren't able to be stolen.
I just tested it with max Thievery. 1 per store seems to be the limit.
Alas, the Diamond Crowbar is completely unstealable.
Is that a bug or something or is it intended to be unstealable? I was able to steal some chain armor from the blacksmith.
Also, is it meant to be impossible to steal from Jerrod? His "steal failure" dialogue is non-unique ("watching you intently") though he seems to be impossible to steal anything from. (even though he's only got one item really worth stealing instead of buying)
I also, I appreciate the stealing dialogue for taking a lone artichoke.
The unique dialogue steal dialogues are really nice though there are only a handful of em.