Who knew that there was such a wealth of statistical data published about traffic flows in NYC? You can't yet get the count for every corner, but if you're interested in people coming in and out of Manhattan, you're in luck.
The first place to look is the The New York Metropolitan Transportation Council (NYMTC) Data and Model page. Most detailed is probably the Hub Bround Report which goes into ridiculous detail about, unsurprisingly, people entering the Hub (Manhattan below 60th street). On the average business day in 2003, how many people entered Manhattan on the N train between 9 and 10AM? 10,031 of course. On the L train? 21,336 (which, if you recall from this chart, is about a third more than a 7 lane freeway).
The NYC Department of Transportation Publications Page is also a good source -- New York City Bridge Traffic Volumes, Bicyclist Fatalities and Serious Injuries, and more.
Gee, all this would be really useful if you were, for example, trying to build a rough model of the effects of a London-like Congestion Pricing scheme for the NYC core...