Initializing/updating/displaying counter variable

Initializing/updating/displaying counter variable

I have a feeling this is simple, but I'm new to Deluge and can't figure it out. 

I'm taking reservations for an event and want to show the remaining spaces in a disabled field called Spaces_Remaining. How/where do I initialize Spaces_Remaining so that it isn't reset to it's initial value each time On Add>>On Load?

Any help would be appreciated.

Bryan