Or 16 with 6x4.5. But the area of film you're paying for is roughly the same. And many films are close to the same price in 120 and 35mm. e.g. Ektachrome* in 120 is also $99.99 for a 5 pack, so $20 / roll, while a 35mm roll is $22, just slightly more expensive. Fuji Acros II is exactly the same price for a 120 roll vs. a 35mm roll ($12.99).
6x9 yields 8 shots; 6X8, 9 shots; 6x7,10 shots; 645, 16 shots. But 6x9 has twice the surface area as 6x6 cropped down to rectangular 645, so enlarges significantly better. 35mm is not even in the same league when it comes to enlarging, but has its own logistical advantages.