I'd like to know how anyone can say with certainty that there is a 'rape culture' when no-one can even agree on how many rapes occur
That question, sincerely asked, is proof of the existence of rape culture - the questioner belongs to one.
Those outside of a rape culture have no real problem recognizing it. US prisons often harbor rape cultures, right? Do you have to count the rapes accurately to know this?
If it is possible to get into an argument over whether some completely and accurately described act was actually a rape or not, and if it wasn't the perp is exonerated and the victim is at fault, you have a rape culture.
If the boundary line is blurred or outside the potential victim's control, so that a potential victim either cannot know or cannot control in advance exactly what their culture will or will not blame them for, you have a rape culture.
If there are cultural norms or established, even named, social customs by which some people accept degradation and diminution of their lives as a precaution against being raped, you have a rape culture.
And so forth. It's not that subtle a matter. Is it known and accepted in one's culture that some people have to routinely worry and take precautions against being raped by members in good standing of that culture, who can expect to come to no serious harm themselves?
Perhaps those who argue that a 'rape culture' exists should first come to some consensus on the risk of a woman being raped, and explain what level of risk is indicative of a 'rape culture'.
A rape culture exists whenever the source of the threat of rape is an ordinary and accepted part of the structure of one's society, jokes about rape and other casual references are common, norms of behavior include recognition of the ordinary possibility of being raped by members of the culture which includes those norms, etc.
As with lynching, "disappearances", etc, it makes no difference whatsoever how many rapes are actually accomplished. Maybe none, maybe just threats and punishments for not taking precautions, and jokes about one's potential victim status.