Whatever the number is, it's simply not enough and plans are underway to expand, even after the last plan was cancelled by the NJ governor because of budget issues.
Which makes a ton of sense, really. Manhattan is a crowded, congested, expensive place with high taxes and high costs of living. Regional transit accessibility is one of the few reasons people would locate a business there. Manhattan is clearly the primary beneficiary of the tunnel, and why New Jersey ever offered to pay for it in the first place (to the tune of ~$9 billion, before the inevitable overruns) is beyond me.
I think NJ is a pretty large beneficiary as well; a huge part of New Jersey's economy is dependent on para-NYC activities, ranging from businesses with satellite operations, to infrastruture (e.g. EWR), and property/income taxes from people who work in NYC and live in NJ. Without all that, cities like Newark would be even worse off economically than they already are, and there wouldn't be a lot going on in, say, Hoboken.
A short commute from New Jersey to Manhattan would convince people that would otherwise live in the city to move to New Jersey, moving their 10% state income tax with them. (And don't forget the city tax. I'm surprised we don't have borough and block taxes yet!) It seems logical that New York would want New Jersey to spend the money to enable this.
