Yeah, if you set up an account at mitelcel.com (which may need to be done in Mexico since they send confirmatory text messages*), you can set up an automatic 50 peso monthly recharge and not only keep your number but also have enough credit in your account to be usable. You *might* be able to set up an account from within the US. One tip is that debit cards don't work (even if they work as credit cards). Up to a point, expired credit is magically resurrected each time you recharge. For example, 50-peso recharges only last for 2 weeks, but my monthly automatic recharge ends up never wasting any credit because it's brought back to life with each recharge. 20 pesos is the smallest recharge you can do, and expires after a week.
Telcel is infuriatingly vague about how long you can let a SIM go unused without using your number. It's not official, but most people report they can go up to 6 months. I haven't heard anyone say they've been able to go longer.
Unless the spare SIM cards are registered and have their own numbers assigned to them, they're useless. In any event, they'll expire if unused just like your "main" SIM.
*For example, Sprint will only unlock the SIM slot for international use, so within the US my Telcel SIMs don't work. A phone bought from Telcel will probably roam fine (but expensively) in the US.