At idle, your intake manifold is creating sufficient vacuum to pull in hho. Your pulling about close to 10-11 psi, the base being 14.7 for atmospheric pressure. At WOT,or acceleration, you pull less vacuum, about 3 psi. In my own opinion, this is still enough to pull HHO, which is really light, with ease. SO I believe one would be sufficient. Although if you produce enough and add two, it would be distributed better. Youd have to be generating a lot of HHO though. Thats just my $.02.
1995 Chevy Camaro
17 MPG base city/highway
Listening to Van Halen