With SBS Essentials there are some specific rules:
1) Thou shalt have no more than ONE SBS Server
2) Thou Shall follow ALL the Wizards
3) Thou Shalt have no more than 25 users
4) Thou Shalt have no more than 25 concurrent connections.
Now this is all fine until you hit an error implying that you have run out of users –
 111202.065215.3944: ClientSetup: JoinDomain failed:
System.ComponentModel.Win32Exception (0x80004005): Your computer could not be joined to the domain.
You have exceeded the maximum number of computer accounts you are allowed to create in this domain.
Contact your system administrator to have this limit reset or increased
 111202.065215.5348: ClientSetup: Exiting JoinDomainTask.Run
 111202.065215.5348: ClientSetup: Task with Id=ClientDeploy.JoinDomain has TaskStatus=Failed
At this point you count the users (in this case 20), then count the machines (in this case 16) then look at the NAS boxes (3) and start to panic.
After a bit of digging we found that there is another limit in windows of 10 users – which is tied into Active Directory. So when a user tries to take ownership of an 11th desktop the error comes up. In 10 years of installing SBS and servers we have never seen this error, but if you use the same user to add all the workstations to SBSE then you very well might.
The solution is to open up the ADSI Edit MMC Snap-in and increase the value of ms-DS-MachineAccountQuota for the domain.
This is a change from SBS 2003 where the wizard created the users on the server, and 2008 where the wizard running as the administrator account creates the user.