Yes it is possible. You should do that in the reporting module.
You need to select the Milestones as the data set, and filter properly by Milestone that is executable(hence all its predecessors have been completed) and by any other filter that would make sense (specific project type, PM etc.)
Once you do that you should create the chart and put the milestone name as the X-Axis and the number of milestones (which is directly correlated to the number of projects in the Y axis).
I would suggest go here: https://success.clarizen.com/hc/en-us/articles/205655178 for additional training on our reporting module.