Drivepool and Drive letters - maximum number of drives?

Hi

Just trying out Drivepool and first question i have is Drive letters and what happens when you run out of them

I am building a large storage/backup server and its likely i will add more than 20 hdd 

Just experimenting with 4 hdd plus OS drive - created a pool which is assigned a drive letter but also the four drives that make up the pool are also assigned drive letters

The drives were not assigned drive letters before adding to the pool (un-formatted)

So what happens when you want multiple pools and have a large number of drives - do you have to use mount points?

Currently i am testing on win7 64bit

Any pointers or advice would be good

Thanks