How can i tag each customer sequentially to create an associate number?

Hi,

Im making a shop to sell our non profit association’s yearly memberships.

For each purchase, I would like to tag a new customer with a sequencial number so that our members have a unique ID. Shopify Customer number is too big to be useful so it is a no go.

Im open to other solutions that do not require the tag, but it is importante that a new customer has a unique number assigned automatically after a purchase. Old customers keep their previous number.

Joao