>If i sell a software created with VFP9 and Mssql to 10 customer, is we
>required to purchase separate license for each customer ?


If you are using a fee-based version of MSSQL, then yes.

If you are using the free version, then none of them have to buy it of course, but they still all need to have it installed somewhere that's accessible. Generally speaking this is one install per customer.