Is there a way to track how often users are logging into a site?
Not built-in, but you can quite easily create a custom module. Leverage our custom tables API or create a custom class of your own. Then hook up to a global event (User logon) and have your code perform an entry whenever a user authenticates. Then you might go ahead and create a Report ouf of the data.
I'd highly suggest following Velem's suggestion of creating a global event. It works very well and can be done in about 20 lines of simple code.
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