I usually try to launch ~2k RPM on my car, don't tend to slip the clutch much, and just modulate the throttle in 1st gear since on anything less than R tires [which I've only been able to run once] on my car, 1st gear is so short that it'll break traction above 3k RPM.

So like everyone said, it depends on the track, car, and tires. Just play around till you're comfortable and pleased with how the car reacts.