To restock a limited edition print, open Edit Options → Edition sizes on your product page and raise the edition cap to your new target number.
Raising the cap to restock a limited edition print
You can freely increase the cap to extend a successful release. Don't lower it below the number of copies already sold, though — that would orphan existing edition holders and break the numbering for people who've already bought.
Syncing the new stock to your store
Once you've updated the edition size in creativehub, re-import the product to your store (Shopify, Etsy, and so on) so the new stock count syncs across.