Hi.
I strongly recomend to use one DB. Why:
- many structures of data are common for all customers, it's no use to replicate it. One job for many future customers. Every common structure with "customer_id" field, and index for this, no efficiency problem
- future changes for structures - one change vs many the same changes for all customers
- new customer = new customer_id, it's no use to create next db
- one DB = easy backup