My cat is 12 and has been sneezing blood I'm wondering if it could just be allergies he has lost most teeth had trouble walking but I think he was dehydrated What does the blood mean

Respiratory infection, nasal polyp, tumor or mass, there are many possibilities.
Unfortunately we can't diagnose on line. You need to get him to a vet, not only to address the bloody sneezing but the poor dental health and trouble walking also.
A trip to the vet is your only option.