Most recent estimates say there are about 39 trillion bacterial cells in the average human [1]. There are slightly less (around 37.2 trillion) human cells. This bacterial community starts from the bacteria passed from our mothers during birth, and spreads around the body, with particularly high concentrations in your gut and on your skin.
Often this situation is framed as 39 trillion bacteria living inside of a human, but we could also consider these bacteria part of the human, since they are essential to our lives, reproduction and identity. Without our microbiomes, which include an uncounted number of archaea, fungi, viruses, and other eukaryotes on top of these bacteria, we would die, since we would be very susceptible to disease and wouldn’t even be able to digest our food [2]. The composition of your microbiome influences your reproductive success [3] and mental health [4]. Microbiomes play a role in human sociality, influencing who you are attracted to [3]. Many gestures of affection and trust (physical contact, kissing, sex, handshakes, etc.) may have evolved because they require exchanging microbes [5].
It makes sense that microbiome exchange is part of social and sexual interaction, since you can tell a lot about a person from their microbiome - their diet, health, and what microbes they will pass on to potential children. With this view of human identity, we see that a human is not only a single being, but a multi-species ecosystem of microbial and human cells living cooperatively.
[1]https://doi.org/10.1101/036103
[2]https://www.nature.com/articles/s41467-020-18983-8
[3]https://www.sciencedirect.com/science/article/pii/S0169534719303258