Names on Every Visit

A gym records each class visit by member number. Member details live in a second table, exported from an old system.

visits:

members:

To put a name on every visit, the manager runs:

Remember exactly how merge lines rows up: it takes each row of the left table and pairs it with every row of the right table that has the same key, writing one combined row for each pair. how="left" also keeps any left row that has no match at all, with NaN in the right table's columns.

How many rows does joined have? Enter a whole number.