Usrs Limit With A Server 2008 R2 Setup As Workgroup

[aqualab]

Hi, I'm setting up a 2008 Server r2, and I would like to use a workgroup insted of a Domain since I have 5 computer with windows 7 home premium. The total of computer is 13 , so is there a limitation of 10 like with XP, or if the 15 cals I have will give me access to the server with all the 13 computers at the same time

thanks and sorry , english is not my first language