Installing a 50 Amp breaker for 220 circuit, using a direct burial cable for 100' for use of electric 220 welder
A welder is defined as a non continuous load so the wire can be sized at 100% of the listed nameplate  per table 630.11.A. But a 50 amp breaker you would need at least #6 copper. I would upsize a bit and put a sub panel in so more could be done than the welder only.