Add a fields named My20 as a Yes/No field. Also add a field if you do not
have it to know First In - like and autonumber.
Use a form for your data entry and on close run a macro that runs two update
queries. The first query sets all My20 to No ( zero ). The second, sorting
descending the autonumber, top 20 update My20 to Yes ( -1 ).
All of your other queries need to have criteria set for My20 field to -1.