Here is one way --
Copy the table structure and data. Open in design view and add an
Autonumber field and change the New Values from Increment to Random.
Create a make table query and sort on the Autonumber field. On the icon bar
change the "All" to 50% and run the query.
Modify the table structure from Autonumber to Number - Interger. Copy the
table structure to ne table.
Create an Unmatch query using the starting table that has the Autonumber and
the table that has 50% of the data. Append the unmatched to the empty copy.