That's not a rate. A rate would be 1 in a million per year, or per generation. What is the time increment?
That's what the problem says...
Presumably one per million births, though I expect the number of non-fatal mutations to actually be much higher (though most of them aren't apt to be of much consequence).
It would seem that in a field of 24,000 antelopes there would be approximately 12,000 births per year. So in 1,000 years there should be 12,000,000 births. If mutations occur at a rate of one per million births, that would suggest 12 mutations.