Then I would use FloorNumberCounter:SpaceNumberCounter
The colon here is key to avoiding the zone id matching other number series that might be found in the vertices. Ideally the zone name would be completely unique such that, if I used your tool to generate an idf, I could run a find and replace routine for your zone name to replace with my own zone name without ruining the file.
One example of a software tool that ignored this is designbuilder. When it exports an .idf, it gives the constructions numbers like 1. If you decide that you would rather it be named something like ExteriorWall, well it certainly doesn't work to replace all instances of 1 with ExteriorWall