It rather depends upon the plants. In most of my tanks I start with around 2 inches of rinsed sand level, then when I aquascape the sand will get pushed more to the back and sides. A depth of 1 inch is fine along the front. If you intend larger sword plants, which have very extensive root systems, you might want a bit more, at least where they will be.
The tank size isn't mentioned in this thread, so difficult to say how many bags. Play Sand comes in 25kg (= 50 lbs) bags for a few dollars. One of these will do a tank up to approximately 30 by 12 inches surface area. It never hurts to have "extra" though, as you may find a need for a bit more here or there.
Byron.