They'll build more than that, but they have to sell over 420 just to break even (according to their numbers) - probably double that to get a decent return on their investment. I wouldn't be surprised if the real numbers aer 500 and 1,000.
For perspective. Boeing didn't deliver its 500'th 747 till 1981 which was 12 years after the first one was delivered. The 1,000'th 747 was delivered in 1993, 24 years after the first delivery. If Boeing sells 500 747-8's both the freigher and passenger models, it will have been an enormous success. I wouldn't be surprised if ten years after the first 747-8 rolls off the line, Boeing announces it is developing Y-3, their replacement for the 777-300, 747-400, 747-8, and A380-800 based on technologies developed for the 787 and 737 replacement. Such a plane would be structurally much lighter than the A380 or 747 and have twin engines. The massive improvement in CASM and cost per ton mile would dry up orders for the A380 well before it reaches a rate of return on capital equal to US T-Bills.
I wouldn't be so quick to count it out. If you think it'll be cancelled after seven or eight aircraft are produced then you must not understand socialism. Socialistic processes are neither motivated nor demotivated by the presence or absence of profits.