Can I have multiple TN visas (at the same time?)

I am presently working in Southern California as a Registered Nurse on a TN visa.
I have been offered temporary part time work with 3 casual staffing companies.
I have a few questions
Do I require a new TN for each of them? I've read yes on the this posting here.....so i think i have my answer there.
Can I have 4 TN visas? is there a limit???
Can I apply for all 3 on one border crossing? Or is this pushing my luck??
If they deny me one or all of these TN's will it affect the TN i am presently working on?
what about the san diego/ tiajuana POE (have you driven across? walked across?) Had success with TN's there?

I would really appreciate any and all advice on this
Thanks so much
Deb

You asked this already!

I simply reposted it as a new posting as no one had answered it when i posted it as a reply in a similiar post. I was just trying to obtain the information , didnt mean to annoy anyone by having multiple posts. I apologize

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I'm a Cdn RN--but married a yank and did my schooling down here. have you checked out allnurses.com lot of good info can be found.