I will be absolutely shocked if you can keep your temps under control running 1800 watts in a 4x8 tent.
Assuming you can control your temps I would do around 20, honestly I have found that plant quantity is not really a big deal, your yields are mostly determined by your lighting. Methods like topping and fimming can cause your plants to have several cola's which will often increase your yield. 1800 watts as a first time grower you should shoot for roughly 900 grams dry which would be 31.5 Oz of bud, aka a little under 2lbs. As you continue to adapt your methods and gain experience growing you will be able to roughly double that.
Make sure to run those lights with the tent closed for several hours all at once before you get plants in there to make sure you can handle the temps. If they exceed 85F you will likely have airy light buds and a lower overall yield.
Also I would top your plant roughly 2 weeks before you intend to flower them, it can take that long for the plant to fully recover from the shock of topping and I have heard of plants going hermie from being overstressed.