I have set of rest apis which are called from mobile apps(Android, iOS).
Is there a way to track these api usage at the server side usng Piwik.
I want to know how many android vs. iOS apps are calling api.
Yes it is possible! You can call the Tracking API from your API to record the data: Tracking HTTP API: API Reference - Matomo Analytics (formerly Piwik Analytics) - Developer Docs - v3