Finding the Next Available Number 
Use
When you create a project definition, WBS element, or project version you can have the system suggest the next free project number or project key.

If you are creating a new WBS element, you only need to enter its level and description in the WBS element overview. As soon as you confirm, a dialog box appears which you can use to find the next free number.
Procedure
A dialog box appears.
Depending on the chosen search strategy, the next free number starting at either the start value or the finish value is suggested.
By choosing Find next you can have the system suggest further numbers.
The system adopts the number.

Projects P001, P123, and P126 exist in the system. If you select Next open, the system will propose P002. If you select After last, the system will propose P127.