unfortunately, there are many, many bots.

Bot spam on Tumblr is rampant even if you don't have many followers or post much.

Anecdata: I have few followers, directly post maybe once every couple of months if that, reblog a few posts a week, and I have no problem with bot spam posts - can't say that I even remember seeing one.

Bot spam followers on the other hand, yeah, a couple a week, but since I don't follow or interact with them, there's no issue.

Around 80% of my (admittedly very small number of) followers are bots, mostly poorly masquerading as women, and advertising porn sites.

They usually seem to unfollow after a week or two though, especially after I block them.

There are bots, but at least for those who (like me) don't have a lot of "real" followers, they're pretty easy to catch, report, and block when they first follow you. (Having never had large numbers of followers, I can't speak for the experience there.)