I always used bottles in multiples of 2 (2, 4, 6...) Place a check valve after each bottle, so you can take it off the system without losing pressure in the tubing.
I ran mine in sets of 2, so that I could alternate replacing one set on a weekly basis, allowing each set to work for 2 weeks.
This gives you no lapse in time without CO2 - a main cause of certain types of algae (brush and hair algae come to mind)
Lukewarm water - not cold, but not quite warm. It refers to the temperature of water, rather than a "type" (assuming 'type' is the right word there)