# of runs depends on a lot of things. Smoothness of the track, smoothness of your driving, heat of the track, complexity of the track, your setup, what speed motor you are running, etc, etc. Ive found sorex tires to not last as long as other tires in less then ideal conditions. It sounds to me like you are just getting started and aren't the fastest guy. In that case I would run them until they wear through. You wont be the fastest but you likely aren't even with new tires (no offense, 90% of us fall into that category).
It sounds like you need to work on the setup of your car. If someone else's car is easy to drive and yours isnt then its likely your setup. You might have too much steering which is whipping the end of your car around. Find one of the helpful fast guys at your track and they should be able to watch you drive and suggest changes that will help.