First look at these two previous articles about customizing popular posts gadget:
Shuffle Blogger popular posts and
Excluding specific popular posts in Blogger.
If you exclude posts from popular posts gadget, then you don't have exact count of popular posts – the count depends on how many excluded posts were in the list.
This hack is installed on this blog, I use 10 popular posts, from which I exclude the current post user is viewing, and some others. Then the list is shuffled and limited to 8 entries. Modify the popcount variable in the script to get a different amount of posts. In my MS-potilas blog I use five popular posts, picked from ten posts minus some possible excluded posts.