Well, let's see.
If 1/3 of those on this list regularly read their GNT as their main NT, that would be several hundred.
At SBL/AAR close to 10,000 people come together, mainly religion and biblical studies profs. I would guess that 2000 probably read the GNT as their main NT. One could add the 1500-2000 at ETS and guesstimate another several hundred.
From these first 2-3 sources, let's say 3000.
Then one must extrapolate that most Greek teachers around the world do NOT attend SBL etc., so my guesstimate goes over 10000.
Add to that all of the students of classics of a generation and I would guess that any who reached a level of, say, a BA would intuitively approach the NT in Greek. To all of those, I recognize that there are little unpredictables, people tucked away in a cabin somewhere, or a librarian, or Christian missionary, who add to the number. So would 20,000 regular readers of the GNT be too small? Probably.
Then, as mentioned, one must consider the Greek nation, especially those trained before 1980. I remember sitting on the beach at a restaurant in Patmos a few years ago, eating Greek yogurt with walnuts and honey, sipping ELLHNIKO. I was reading THN APOKALYPSIN. I mean, what else, while on Patmos? The young waitress asked what I was reading, seeing this English-speaking couple looking at a Greek text. I mentioned H APOKALYPSIS and she commented that she had just read it and we discussed a point or two. Apparently she read it in the 'old language' (and text/KEIMENON, probably 1904), because that is what we discussed (though technically I would have had NA 27/UBS4 open)
. That was a visit to Patmos when I later got to listen in and address a Greek highschool class, where smiles and eyes rolled hearing this foreigner use such oddities in live speech as the old 'future' and 'APAREMFATA'. It was fun.