That’s a great question, and not an easy one to answer. We can infer a historical typical number of partners per mating period (periovulatory period) across species of great apes (humans, chimpanzees, bonobos, gorillas, and orangutans). We’d like to know both how much competition there is for mates before copulation, and how much post-copulatory competition (sperm competition) there is – two very different questions. If we just focus on how many partners are typical per mating period, we can infer that from different physical features. That gives us expected relative number of partners, with chimpanzees and bonobos having many at one time and gorillas, orangutans, and humans, having just 1 or two at a time. But, this doesn’t tell us how many partners each species had sequentially (e.g. was it the same one or two for each mating period, or did it differ?).
Also there are huge differences across human cultures that make modern estimates much more difficult. That said, I also wouldn’t use historical estimates as any way for how we *should* be living.