All item in envato market is a one time payment for license. but if you need author support after expired author free support (first 6 months) then you will need to purchase/extend author support.
for each purchase you will get one lixense for one end product
yes, you can purchase a single license
If you want to make multiple end products then for each end product you will need individual license. So, you have to purchase individual license for each end product
for any technical query you can or pre-purchase query you can contact the item author and ask them.