As an inveterate counter of mites, I would say your "lots" is probably 300 - 500, which is a hell of a lot mid-winter! You've done a second count so I'd be monitoring that daily to see what is happening to the infestation. If by the first or second of January you are still getting a drop of in excess of 5 per day, I'd certainly do the third vape.
On the one hive that I've had a significant drop after the first vape on 17 December, it dropped 250 in 5 days, I vaped on day 6 and it dropped 130 in the next 6 days and if it ever stops raining, I'll go out and do a quick count to see if I'm on top of the situation after the 3rd vape yesterday.
It's a lot easier to count if you draw lines on the monitoring board to form rectangles about 75mm square. On a really heavy drop, count each square and jot it down. When you've counted all the squares, just add up the numbers and Robert is your Dad's brother. The reason for counting each square separately is that, if counting a big drop, you are interrupted, you don't have to start from the beginning again - been there, done that. I've found that mites are not uniformly distributed across the monitoring board so only counting a few squares and inflating this number in proportion to the total number of squares will not give an accurate mite count.
Some people will say "does it matter?" but as my old man used to say, "if a job's worth doing, it's worth doing properly". Instead of reporting "a lot of mites" dropping and after treatments you had "quite a lot" and after the 3rd treatment there was still "quite a few", you can report numbers. You never know, one day some clever mathematician might come up with a model that replicates the fall of mites after a treatment and she will need our figures to authenticate her work on the mathematical model. Besides, you can draw pretty graphs of the drop, if you know the figures!
CVB