I'm not an server administrator, but i would like to know why on the server i have like 10 myslq instances? Is it good or bad? Does it negatively impact speed of my website?
By default MySQL starts more processes ("threads") to improve performance. If you are familiar with Apache, it does the same thing.
If you want to limit the number of threads, you'll want to limit the number of connections. You can use the max_user_connections variable to do this or with recent versions (5.5+) you can limit them per user.