How do you know what gear to buy to match to the gear on the crank shaft?

The stock cam gear is fixed to the cam with rivets, and generally speaking not replaced. You need a new cam and lifters, which depending on your engine is probably a good idea if you've got the case split.

I have the new cam from webcam and their lifters. Is there different pitch to the gears that need to be matched to the crank gear or does the standard gear work for all?

Webcam offers a new cam gear, only 1 version used.

Factory cam gears came in different +/- pitches to compensate for tooling wear during production. The replacement new gears are only available as a zero gear. Most of the time the zero gear works.

When I install a cam I install the crank and cam (nothing else) I turn over the crank and look to see if the cam lifts out of the DT bearing saddle, if loose I check for backlash. If it lifts out I put one finger on the DT cam journal, turn the crank, if it stays put with light pressure it's still OK.
The very odd time I need a +/- I pick one from my stock of used gears, which I have to modify (pump/cam bolt clearance) for the aftermarket cam.
I rarely have to do this, but I have a T1 on the bench right now that will need a modified used minus gear.

Mark Henry knows his stuff!

Thanks, that's the info I needed.