Ummmm, for me, it was one partner per year, starting at almost my 18th year. I didn't have sex at all until about a week before my 18th birthday, and until I was nearly 20, I was lucky if I had sex more than twice a month. I was interested in having sex more often, but I usually didn't have the opportunity with my partner of choice to have it more often. I got married just before my 20th birthday, and being young newlyweds, my husband and I would usually have sex at least once a day for most of the month, unless I was having my period (which squicked him out at first) or unless he was temporarily stationed in another country (in which case again, I didn't have sex at all). This continued until I got pregnant three times in two years, using birth control each time. After the third pregnancy, I quit having sex at ALL until I could get my tubes tied, so for about a year and a half there was another dry spell.
It's really going to depend a lot on the individual female. Some women never have sex. Some have frequent sex. I doubt that most women ever have as many as 20 partners over their lifetimes, unless they are actually sex workers. It's also going to vary by age (I consider 16 to be too young to be involved in a sexual relationship for most girls, and I don't care if it's legal or not) and whether or not she's in a sexual relationship with someone. If a woman doesn't have a boyfriend, or girlfriend, and isn't interested in casual hookups, then she's probably not going to have sex at all. If she's a swinger, is interested in casual hookups, and otherwise has available partners, then she might have many sexual encounters in a month.
"Normal" covers a very wide range of behaviors. However, your guesses are wildly over the average range, except for a woman in her early to mid 20s, who might be having sex that often.