That is a loaded question like asking how many welding processes there are, but I'll try and help you out abit. Alot of states have welder qualification tests which carry minimal weight in the construction industry. Most all reputable mechanical contractors have their own "in-house" certifications which cover them to work on piping systems and are santioned by the ASME.
A standard welding test for pipe welding carbon steel and will allow you to weld 1"-sch-40 pipe up to an unlimitted diameter uses 6010 rod for the root pass with the balance using 7018 electrodes on 2"-XX heavy carbon steel pipe in the 6-G position.
Stainless steel pipe uses the GTAW process with 308L rod on schedule-40-2" diameter pipe in the 6-G positiion and an Argon backing purge.
If you were going to weld an alloyed stainless steel, say nickel-chrome(Hastelloy) you would have to test on that material and because it's so expensive(the material) only seasoned welders will even be considered for the test and would be job specific.
Welder qualification is pretty indept especially if the ASME is involved. Generally specking I'd guess that there are 50 or so piping certs you could get, but like I said, they would be contractor specific and here in the USA, you generally have to re-test if you have not welded that procedure within a 6 months time frame. You will spend a life time trying of get all of them.