Must I repeat the prayers after touching the vulva when applying cream, without knowing that this nullifies ablution, and how can I repeat them if I do not know their number?

There is no need to re-perform past prayers for someone who was ignorant of the ruling on touching the private parts. If they are re-performed as a precautionary measure, there is no harm, as the issue is subject to scholarly disagreement. What is obligatory for the future is to perform ablution after touching the private parts.