How to get a number of instances into overview graph?

The only way I know of to see this is to display the legend. 

Some DataSources have used some hakey methods to create an additional "instance" that tracks the same stats in aggregate for all instances. You could theoretically do this and have instance count be a metric tracked on that instance. Technically, it would be tracked on all instances and would be 1 for all other and would be the number of instances for the aggregate.

There's a feature request submission option right in the product. Additionally, you can post to our a href="https://communities.logicmonitor.com/forum/10-feature-requests/" rel="">Feature Requests category to gather interest from other customers (which helps prioritize features in the backlog).