As others have said, it's totally up to you. When I first started collecting T206 backs in the early-to-mid 1990s, pretty much nobody paid attention to factory numbers. Even Piedmont Factory 42 didn't really command a premium, or maybe just a small one, as far as I know. So when I put together this page around 1994/95, I considered the Piedmont, Sweet Caporal, and Sovereign parts of my back collection to be complete at a 150, 350, and 460 of each:
When I got back into T206 more seriously in the past year or two, factory numbers had become an important factor in most people's definition of a back collection (or back run of a single player), and I began to pay attention to them too. I looked through my Piedmont 460s from back in the day and found that I didn't have any Factory 42s, but I was able to pick one up from Ted Z at the National this year. Then a few months ago I did an inventory of my T206s by backs, including factory numbers, and found that I had all of the Sweet Caporal backs except for 460 factory 42, no overprint. Luckily I was able to find one almost immediately, and I didn't have to pay any kind of premium. (I just recently got another one, and didn't even realize that's what it was until I got the card in the mail and looked at it closely.)