Figure out the largest footprint that light can provide. Make sure the plant is about half that area before it goes to bloom and you should be okay.
it's never about how many plants you grow. that's incomplete information. how large will they be? Well, that should be dependent on the space you have and the footprint of your light, then work backward from there.
20k lux means different things with different lights... it will tell you intensity of light in a proportional way, so you can use it for a good function. If you see any stress at 20k, you can start a tad further away or dim it next time. further away increases footprint but reduces intensity in any 1 spot of that larger footprint (same number of photons, spread out) - a happy medium is usually found.