How many freewheel thread sizes are there?

The ISO freewheel thread standard is 1.375" x 24tpi

Most freewheels I have encountered have been ISO.

Some hubs use "British" threading at 1.370" x 24tpi

If you use an ISO freewheel on a British thread, it will work, but you should check there is enough thread engagement to avoid stripping the hub. The trials-oriented retailer TartyBikes suggests 9 thread is enough (Trials bikes often run a freewheel at the crank using a 1.370" x 24tpi thread)

Italian freewheel thread is 1.378" x 24tpi: Fine on ISO, but even shallower thread engagement on British hubs than ISO on British. Still viable though.

French freewheel thread is 1.366" x 25.4 tpi or 34.7 x 1mm

If you tried mixing 24tpi with 25.4tpi at the same diameter, the threads would be strained and with enough engagement would eventually jam. As the French diameter is smaller, an Italian, ISO or British freewheel will just about thread on but will have very little thread contact and will slip under load, blunting the hub threads.

Metric BMX is M30 x 1[mm] or 1.181" x 25.4 tpi. This is a small enough diameter that all the above sized freewheels will be obviously sloppy on an M30 hub.

I've never seen an M35 x 1 freewheel, but it seems close enough to French that it should work.

Mini-freewheel (Which I've seen on some DMR Revolver hubs) is 1.18" x 24tpi

Mini-freewheel and Metric BMX are close enough to cause trouble. The diameters are within tolerance but there will be strain on the threads and the hub threads will be less snug if you later replace the freewheel with the correct item.

Praise be to Sheldon.