The referential Blogger Help: What are the limits on my Blogger account? tells us
Team Members: There is a limit of 100 members per blog.A member can be either an administrator, an author, or (if the blog is private) a designated reader.
In the help forums we have a dialogue where Blogger Employee admitted to a recently imposed limit per blog, of 100 Administrators / Authors / Readers, to reduce server overload.
there were lots of performance issues that arised as a blog amassed more than 100 readers.
If you have a private blog, you'll be able to have a maximum of 99 readers, since every blog has to have at least 1 administrator.
And even as severe as the limitation of 100 seems to some, other blog owners would be happy to be able to enjoy that number.