This plugin will show the number of subscribers according to FeedBurner/Google. Important: WordPress 2.7 and up.
If you have a widget ready dynamic sidebar, you can add the FSS Widget.
Add this code: [fss_subscribers] to view the subscriber count.
You can customize how this looks for example:
There are [fss_subscribers] subscribers to this blog.
There are 20 subscribers to this blog.