How many calories can you burn doing 75 knee highs?

Not any important amount.

When it comes to burning calories, how hard and for how long is much more important than what you do. Pretty much all activities that gives you the same increase in heart rate for the same time will burn the same amount of calories.

Now, 75 knee highs won't take that long to complete. Maybe 1.5 minute if we're generous.

And it's about as strenuous as aerobics.

So if one hour of aerobics would use up 450-600 calories, 1.5 minutes of knee highs will use up 11-15 calories.