How many molecules of lactate can one molecule of lactate dehydrogenase act on in 21 minutes?

The turnover number (number of molecules the enzyme can act on per second) for lactase dehydrogenase is 1,000 lactate molecules per second.
In one minute, this becomes:

(1,000 molecules / second) * (60 seconds / minute)
= 60,000 molecules / minute

In 21 minutes, this will be:

60,000 * 21

= 1.26 x 10⁶ molecules of lactate