I had hives last night and took 2 benadryl (diphenhydramine) one at 12 and one at 3am. if i get hives could that dose last night subdue new ones today at 4:18pm?

Don't think so: Benadryl (diphenhydramine) usually is taken every 6 hours. It generally lasts 6-8 hours. Other factors include how big a dose was taken, and how many hives are coming out on the skin. For example, having large numbers of hives continuing to appear may mean a need for a bigger dose of Benadryl (diphenhydramine) plus adding other medications.