It will normally take an hour or so for Facebook to fully match customers from your database. During this period it will sometimes show 'Fewer than 1000'. This may sometimes happen with source audiences as well as Lookalike audiences and is normally resolved in a few hours. If your source audience continues to say this after 24hrs, and you are sure that you have more than this amount in your Shopify, it is possible it had a very low match rate. Contact us so we can take a look!