All depends how deep you want to get into it. If you are doing an overlapping seam a 1/4 inch weld every 2 inches will equal most factory spot welding, although the pan was one piece originally with no seams so where do you stop? The more welding you do the stronger it is, it all depends on how nice your welder is behaving and how pretty you want it to look finished. The more you weld the more you have to grind.
Another way to do a strong overlap is to drill 1/4 - 3/8 holes 3/4 in from the edge of the patch and fill them with weld, that way they look like spot welds and hold on better.