Well you spelt "thief" wrong in the title, but you covered it up by correcting yourself in the post, heh. I first clicked on this expecting to see you get flamed but it's np. Anyways, if you're on the Atlantic shard, my guild has a thief training room in our guild house that you can use. I've written 4 books that'll answer all of these questions and more:
How To Train A Thief (40 pages)
The Basics Of Stealing (40 pages)
Thief Macros (13 pages)
The Thief NPC Guild (40 pages)
That's 133 pages in all, and they aren't some crappy books that i just threw together. I've spent a lot of time perfecting these, and even went back and changed some things here and there so they're as correct as possible. But yeah, if you are from Atlantic, or you would like to just make a newb char and come read them on Atlantic, PM me...
P.S. As for your question about how many thieves are still out there, my guild cRaZyPoO [P0O] is the biggest thief guild in UO. We have 60+ active thieves (active enough to show up on the uo site guild search, it shows everyone who has been on within the past 2 weeks or month, forgot which), and we're recruiting new thieves almost every day. Come and join us if ya want.