Here are the Grand Central commuter numbers:
http://web.mta.info/mta/news/books/docs/2017%20MNR%20Ridership%20Appendix.pdf
86.5M per year = 247,000 per day.
_That_ is how many people need to be tested.
I kid you not.
Hopefully, most of the daily 247,000 are commuters like the lawyer from New Rochelle, who live elsewhere and work in the city. Same people every day.