Assuming you have recently purchased the propoerty?
Is it a new build? It's possible the town hall haven't got round to issuing IBI bills for the property yet (that doesn't mean you won't have to pay it!). Do you not have a copy of the last bill paid by the previous owner? You (or your legal representative) should have seen it before purchasing, to ensure IBI was paid up to date.
The "IBI" number you're looking for is basically the "referencia catastral", which can be found on your escritura (or copia simple - the deed). Take that to the town hall to show that you are the owner, and if there is a bill pending they should be able to give it to you.
Whoever is the owner of the property on January 1st is liable for the IBI in that calendar year. Bit daft, as the bills aren't usually issued until the autumn.