You need to have a good enough warm up to feel good about your skiing and confident enough to attack in the start gate.
For some people, that's one run
For others, it's 5-6
Figure out what works for you. If you're a high-rep person, do what you have to to preserve the wax (either leave a bit extra and scrape/brush at the base before your race run or at the start, as @ScotsSkier suggested) or get a second pair of skis (doesn't necessarily have to be a race ski - what works for you?).
But ... if you aren't fully confident and ready to commit in the start ... then I don't care how good your wax is, you're not going to ski well.
Depending on the hill, wax in GS may not be a big factor ...
For some people, that's one run
For others, it's 5-6
Figure out what works for you. If you're a high-rep person, do what you have to to preserve the wax (either leave a bit extra and scrape/brush at the base before your race run or at the start, as @ScotsSkier suggested) or get a second pair of skis (doesn't necessarily have to be a race ski - what works for you?).
But ... if you aren't fully confident and ready to commit in the start ... then I don't care how good your wax is, you're not going to ski well.
Depending on the hill, wax in GS may not be a big factor ...