A 1 mod gear has 1 tooth for every mm of pitch circle diameter. So a gear of 60mm PCD of mod 1, would have 1 tooth for each mm, i.e 60 teeth. A 0.5 mod gear with 50 teeth, would have a 25mm PCD. (Where PCD is the distance between the gear center and the point of contact with a meshing gear). Hope that also helps.
You will find that things get a little more confusing as depending on the number of teeth on the gear, you need a slightly different cutter profile to mesh correctly. This is why for each module, there is a "set" of 8 cutters to deal with all the wheel counts. So to aquire a complete set of cutters, you not only need the module, but a set of 1-8 cutters for that module!
Here is an example of a range of Cutters for Involute gear form pinched off the net:
Cutter Number - Teeth to cut
1 135 teeth to rack
2 55-134
3 35-54
4 26-34
5 21-25
6 17-20
7 14-16
8 12-13
If your interested in cutting clock pinions (which your not as you are specifically mentioning involute and clock cutters take on a cyloidal form) your talking £70-100 per cutter!! So near 600-800 for a complete set .... then the options of all the different modules available...!!!!
Ivan Laws Workshop practice book - Gear and Gear Cutting #17 is amazing when it comes to gearcutting.