How often to I have to refil to keep my number

Often enough to keep a positive account balance. If your account balance is $0 for more than 60 days after service expires, your account is cancelled and your number lost. 

Gretat, so I have to refill at least once every 60 days right?

Thank you!

That will definitely work but depending on your refill amount, you could refill less often. Refills expire if not used. A $10-$29 refill expires after 30 days, $30-$99 expires after 90 days and $100 or higher expires in a year.