I think my dog had two mates

Question
I have a pure breed boxer, I breed her with another pure breed male boxer. But i think that a stray dog might have also got with her. If this is true when my dog has puppies will some of them be pure breed and some be mixed.

Answer
It is possible for puppies in the same litter to have different fathers, yes.  If you are registering the litter with AKC, you will need to do DNA profiling of the dam, the sire you know, and all the puppies, and apply for a multiple-sired litter registration.  (You won't be able to register the puppies from the unknown sire, of course.)  More on this process can be found here:
http://www.akc.org/dna/multisire.cfm