There's no single answer to how many sex education classes public school students receive because it varies dramatically by state, and even by school district within a state. Some states mandate comprehensive sex education, while others only require abstinence-only education, and many have no state-level mandate at all. The number of classes, the content covered, and even whether sex education is offered at all can therefore differ significantly.
It's safe to say that the number of classes ranges from zero to potentially several, with a median likely falling somewhere in the single digits for many students, but this is a rough estimate based on the wide variability. There's no reliable national average that can be given.