Actually they only did 4:
2005 PowerPC based*
2006 Core Solo/Core Duo based
2007 Core2duo based
2009 Core2duo based*
* 2009 had an all new port layout, so it was a moderate redesign. The others were all relatively the same.
At this point we are at the 4th version of this model if it is released this year.
The biggest question around the Mini this year is will it get PCIE based SSD's. Their options:
1. Stick to SATA since it is "only a Mini", but that very well could leave the Mini to be the only computer left with SATA based SSD's (especially if Apple drops the cMBP).
2. Figure out a way to route a PCIE connection using ribbon cables, so that they do not have to redesign the Mini (there isn't room right now to attach it directly to the motherboard as far as I can tell).
3. Completely redesign the Mini, so that it is a bit taller and then it could hang the PCIE SSD off the top of the mobo....
Note: We need to remember that mid-year spec bumps that Apple does are not considered new versions, and often times don't even carry a new model number of if they do they are more "point releases" (i.e. the 2005 early was PowerMac10,1 and the late 2005 was PowerMac10,2).