Hi, Is it possible to have different invoice numbers number for different clients. So for example for client A, A.001. For Client B, B.001; generated automatically

Hi johnadegbile 

From Settings > Account and Settings > Sales you can set Custom Transaction Numbers to yes; then when creating an invoice you'll be able to override/edit the invoice number.  There's no way to auto-set the prefix by customer so that part would be manual.

Hi johnadegbile. Thanks for reaching out to the Community. Just to add to the valuable info @LeithG has provided above, check out this step-by-step article which shows you how to: How to change the invoice number on sales forms. Once you've enabled Custom transaction numbers, an Invoice no. file becomes visible and you'd also have to manually add the letter A or B on the invoice number. 

Let me know if you need further assistance. I'm here to help.