First Tri tomorrow, no race belt, can the number be in the back for bike and run?

Safety pin it to your shirt.

Mine fell off one race, so I just held it in one hand and ran around with it, when people complained they couldn't see my number I just waved it at them 

Well, most races will say they want your number in the front on the run and--if they haven't provided a number for your bike frame and/or helmet--they want it in the back on the bike.  Your call.  Probably more likely to get dinged by an official on the bike if you don't have a visible number than you would be on the run.  (Honestly, I think they want to see your number in front on the run more to help sell race photos than for "official" use.)

You might find someone with a spare race belt if you ask around.  I bring an extra race belt pretty much to be a nice guy if someone needs one (and extra goggles for the same reason).

Do you have any adjustable straps (off a backpack) or a piece of thick elastic?  Add in a couple safety pins and you have a basic belt.  May not look like the fancy $10 ones, but it works.

I just went shirtless, so I didn't attach the number.  They mark your arms with markers.  If you're female, custom would suggest a shirt, though. 

Have any tighty whities?  Chop off all but the elastic waistband.

That is how race belts started.

I borrowed this idea tonight for my Sunday tri. Well, not tighty whities but some Danskin cotton underwear with a wider band. I was actually looking at a black thong and thought, hmm. Then, ah, no. LOL.  Hey, would you really notice over black shorts?

It feels really good to hear your name as you approach the finish line. They can only do that if your number is on the front. I usually make sure it's good to go when I start getting close.