There is no number. 1 is all it takes. Although condoms can reduce the risk of transmission, they are not 100% against HPV. HPV is a skin virus, so it can be carried/transmitted from areas that are not covered by a condom.
As tgrobar said, there is no magic number. Someone can be exposed to HPV at his/her first sexual contact. Sexual contact includes a variety of sexual foreplay, not just sexual intercourse.
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