I started off with one stem that I kept dividing. At the end I had maybe 6-8 closely planted stems, but it grew in a tangled mess with as much lateral growth as vertical and an enormous amount of side shoots.
I think H polysperma is a relatively undemanding plant as far as lighting and CO2 goes. Four 2L bottles should do you just fine. You may want to run your CO2 line through some form of bubble counter (make one out of a 1L water bottle) just to catch any gunk that the yeast mixture is likely to produce. Otherwise the Hagen ladders get gummed up pretty quick which traps the bubbles instead of letting them climb.