I am new to Tumblr and don't know how to tell if someone has read my blog, or how many. How can I see this info?
Tumblr doesn't offer any statistics by default. However, you can edit your theme and add in a tracking service's code that will allow you to see how many people have visited your blog (this would also include your own visits).
Google Analytics is the easiest way to get started and it's free.
To get to your theme editor you'll need to go to
http://www.tumblr.com/customize/<blogname> and click on
Edit HTML to paste in the code that Google Analytics gives you.