I myself have two demat accounts with two different DPs using the same Pan No. there is no restriction on the no of demat accounts that a person can hold even on the same Pan No as demat A/Cs are treated just like bank accounts (and there is no restriciton on the number of bank accounts you can have).

As for IPO application, it should be only one application per PAN number. An individual can legally apply only ONCE in an IPO.

So in short you can have n number of demat accounts with n number of DPs but while applying for for IPOs it is only one application per Pan No.