plugin for calculate field,,

1. how to calculate total no. of of Projects created for that Account.

2.by custom workflow? which set the class field of project based on priority set. User can run this workflow as and when he/she needs.  Below is range of priority for the class.

1-3                  Class A

4-7                 Class B

8-10               Class C

please can some one send me sample code for this.