Which scores count?

While all your scores will be available to the Adcoms when you request your scores be submitted for your application, they should consider only the score you wish (the highest score obviously). In general, they will disregard outliers (low scores) and assume that there were conditions which clearly did not allow you to do your best. When it comes to determining the average GMAT for incoming classes, they will only count your best score, just make sure you note that score on your application.

It depends. The best score counts but because they will see your other scores (and how many times you took the test) that may influence their judgment subconsciously. If you take the test 7 or 8 times, it certainly will raise a flag in their mind.