Not really sure about an activity available in this case there is a work around to get 20 records as follows,
Loop the list of objects , create a variable which holds the starting number as 0 and another one for hold total Number. Create a fresh list to hold 20 records out side the loop. Change loop type to while add InitialNumber<=totalNumber. Inside loop add the current iterator object to new list followed by increase the initialnumber by 1 each time.
Your new list will contains 20 records. In future if you want to make as dynamic , make that total number as constant .
Hope this helps!
Instead of directly adding to a list, you can add that through a loop and have the count =0 and when the iterator increases
you can increase the count and also you can have a flag[boolean value] for the first 20 records
Hope this might help