Np I was in the same position one month ago
Just for some content could be worth
I never needed the extra copy but is better to keep at least 1 if needed
The seconds extra base copy is just so if the legend gets a super evolution you can evolve one of the copy’s to is super evolution
You should only keep the 8 chars for limit break if you aren’t a f2p player
For f2p is way better to sell those extra copy’s for RP and get ships, flags, CC and other rare items
For LB you can use plates
Usopp there is really good to tho
Lb if you can max the char potential abilities till last level for RR and other non legend chars, for legends you should search for a guide on the internet but the basic is keep at least 1 evolution form(super / normal / unevolved form) and sell the rest of the dupes for rp, if the char does not have a super evo yet just save an extra copy. For dupe chars (the ones that will not get a super evo) just sell them if you have the evolved and unevolved form (idk if they could or not get a super evo tho).
If you don't have the resources to limit break a char keep all the copies till you can lb him and then feed him the copies you have. Usually i just lb a char if i have 8 copies of him so i can waste resources and rainbow that char this is kinda hard to understand if you don't know what limit break is, so if you don't really understand lb and potential abilities try to search a guide on limit break so you can understand better what a said above.
Hope you have fun :D