Depends if you want to draw 500mA or 500A, what cells you’re using, what welder you have access to, the shape your battery needs to be, how your series connections will be constructed, what voltage drop and heat level you’re okay with, and a thousand other things.
Generally speaking, 50mm wide 0.2mm thick nickel is pretty good, in the absence of any relevant information.
I think each well-done spot weld is good for about 5A, if I recall correctly. There is a Q/A thread: