How many results are shown by default when using a top or rare command?

The answer is "10".

Rare command showcases the slightest basic values of a field. 
Finds the minimum continuous tuple of values of all fields in the field list. In the event that the <by-clause> is indicated, this command returns uncommon tuples of qualities for each unmistakable tuple of estimations of the gathering by fields. 
This order works indistinguishably to the top command, with the exception of that the rare command finds the slightest continuous rather than the most regular.