Hi Amit,
lets try and figure this out. To get a count it is best to use Wix Data API.
I assume your model looks something close to this - each submission holds a reference to a member who submitted the data (I will call this field submittedBy ). So, if you want get a count of posts by a certain member you would:
wixData.query('Submissions')
.eq('submittedBy', memberId)
.count()
.then(numberOfSubmissions => console.log('Number of submissions: ' + numberOfSubmissions);
You can find Wix Data API documentation here:
Let me know if you have further questions. Good luck!