Eklispe is currently banned for more familiars. Why? Because it ocurred to me as I was watching Date A Live again. (jk,jk)

And there is no rule saying that you can't have more than one familiar. Honestly, I myself hadn't thought about it until now. So, yes, there is no limit to a mage having more than one familiar, but they'd need an adequate mana pool to keep all of them useful.