There has been some confusion as to what number should be put into the MinUserDC registry key if you decide to use it. Most people use this registry key to reduce the load on their PDC emulator (PDCE).
298879 Exchange Server 2003 and Exchange 2000 Server may experience performance problems when the PDC emulator is used for DSAccess
If you read that article it may seem that setting the number to one less than the total number of DCs in the site might be a good idea. But, what number should you subtract 1 from? If you have other DCs from other domains in the Site, should they be included? Will this limit the total number of DCs that the Exchange uses? What if I figure out that there are 10 DCs available and I set MinUserDC at 9? If I have 1 DC stop working, then won't the PDCE be used? Should I set the number at 8 then? But then, what if 2 go down...?
The easy answer to this is: Don't worry about it. If your overall goal is to not use the PDCE, then set the value at "1". The way this works is that as long as there is at least more than one DC available, then we won't ever use the PDCE. Simple enough...
Next time DSAccess goes through and creates the list of DCs available to decide which ones to use, it won't include the PDCE in that list. Unless, of course, there is only one server available then we will use the PDCE if it is the only one still running.
Setting this number at 1, doesn't limit the total number to only 1 DC. (Well, I suppose it would if you only had 2 DCs.) It really is like a switch. If you have the number "X" as the value and we have more than "X" DCs available, the option to use the PDCE is switched off and we don't use the PDCE.
I hope that this makes more sense now.