OOT: Maximum disks in Raid 5

Dear all,Maybe this is an out of topic question, but maybe someone be able to help me.

Right now my domino server run in windows 2003 server

for OS, I use raid 1 configuration with 2 disks.

for transaction log, I use 1 disk, no raid.

for data, I use raid 5 with 5 disks.

because the space in the server already full, I plan to add more disks to the raid 5.

Question:

What is the recommended maximum disks in one raid 5?

I afraid adding more disks to the raid 5 will get me into the troubles.

TIA