lakrisgubben

This plugin uses BuddyPress’s notification system to let users subscribe to all new posts on a site (or many sites if you run Multisite). When a new post is published all users subscribed to the site gets a notification and optionally an email (can be turned off in their BuddyPress email settings).
<h4>Requires</h4>
<ul>
<li>BuddyPress 2.3 or later</li>
<li>WordPress 4.3 or later</li>
</ul>

BP Site Subscriber

This plugin uses BuddyPress's notification system to let users subscribe to all new posts on a site (or many sites if you run Multisite).

Brajesh Singh

This is a plugin for wpmu/wpmu+byddypress powered websites, where network administrators can limit the number of blogs a user can signup. 
It is pretty simple and adds an option to NetworkAdmin->Dashboard->network settings page, where you can limit the number of blogs. No additional frills required.
What It does
It adds an option to the Network settings page when you are logged in as site admin of the wpmu site(or wpmu+buddyppress site),Look at the bottom of options page,and You will see a text box like this asking for number of blogs allowed per user.If you set it to zero(which is the default),It will not restrict the blog registration then. 
For any support or any questions,please leave a comment at my blog
<a href="http://buddydev.com/buddypress/limit-blogs-per-user-plugin-for-wpmu/" rel="nofollow ugc">http://buddydev.com/buddypress/limit-blogs-per-user-plugin-for-wpmu/</a> 
And yeh did I mention <a href="http://blog.strategy11.com/" rel="nofollow ugc">Steph</a> has been very kind in reporting bugs and suggesting fixes.Many Thanks to Steph.

Limit Blogs Per User

This plugin is for WordPress Multisite and/or WordPress Multisite+buddypress based social network.It limits the number of blogs a user can create.

BP Site Subscriber vs Limit Blogs Per User