Followers
On the blog itself. The number a visitor reads before deciding whether your archive is worth scrolling.
3 services available
Services for Tumblr
Tumblr has three services here, and one of them does something the other two do not: a reblog puts your post on somebody else's blog. The pages below carry the rate, the minimum and the delivery window that apply right now.
Why a reblog is not a like with extra steps
On most platforms the share counter is a louder version of the like counter. On Tumblr it is a different object.
A like is a number under your post, read by whoever opens the post.
A reblog copies the post onto another blog, where it sits in that blog's archive with your name attached. The copy is permanent in the sense that it does not disappear when the original does, which is a mechanic almost no other platform has.
Followers are the third thing and the slowest: they are the number a visitor reads on your blog before deciding whether to scroll. None of the three changes what Tumblr shows to anybody.