Common column value for different users

We are working on windows application and there is a requirement that some invoice number is assigned to users like 5 orders are there for 1 user and 3 for other users. The DB for all users are same (on server). when user clicks 'Charge' button then loop executes and generate invoice number, the invoice number is generated by incrementing 1 in last generated and assigned invoice number to any user. Now in loop invoice number increments and assigned to next user say 5 orders then 5 invoice numbers (like 05, 06, 07,08,09,10). Now the assigned invoice number is saved in sql table and last assigned invc no. is picked from there. The problem is that if at the same time 2 users use the application then same invoice number has been assigning to different users.
Please help to find solution for this problem.