If something often happens, it happens many times or much of the time.

You use how often to ask questions about frequency. You also use often in reported clauses and other statements to give information about the frequency of something.

If something happens every so often, it happens regularly, but with fairly long intervals between each occasion.

If you say that something happens as often as not, or more often than not, you mean that it happens fairly frequently, and that this can be considered as typical.