This largely depends on the type of Shopify plan you have. Generally speaking, each store needs its own plan (unless you go with Shopify plus where you can have multiple stores under the same plan):
Basic: $19/month Shopify: $49/month, offers 5 additional staff accountsAdvanced: $299/month, offers 15 additional staff accountsPlus: $2,300/month, offers unlimited staff accounts and up to 9 additional stores