Hello all! 
I have a standard object "Account" with a custom field(picklist) "Stato";
I want to create a custom field "Nome account" where the value is an automatic incremental number that is assigned when the picklist value of "Stato" is changed to 'Cliente',  if that number was not already present.
For example "AC 000000" where "000000" must be an autonumber.

Can you help me?

Thank you very much!