no, they are not limited, but you will need a lot of them later for CC and AW
platinum and above rarity need 3 silvers to CC, example Bashira need 3 archers to CC
you can only get silvers from event with horrible droprates, or via summon
i couldnt CC my Berenice for about 3 weeks because lack of heavy armor, event did not drop heavy armor and summon keep giving me class other than heavy armor