Depends on what you need to do with the value, but you may need to make a call to the REST API to get that info. It doesn't appear that information is available in the Place object that is available to the .header template.
What is your need for the value?
Thanks for the reply.
1. We would like to display the members count of a social group in the place header when any user visits any social group page, Just display purpose we need members count.
2. Please provide the api to know what is the current placeid, so that I can get members of that particular group.
Thank you so much for your reply.