placing wards is a lot about keeping an eye on where the enemy players are, if you just keep one ward in the river and one in the further brush (if you are purple side) or 1 in tri bush 1 river 1 further brush (if you are blue side) That setup will prevent any early ganks.
I try and keep an eye on the enemy jungler/mid, I don't ward all those spots right away because if their team is mostly top, using 2-3 wards bot is a waste. If you know their jungler is nearby you should ward, if you notice the mid is missing you should make sure you have wards up. It is a lot of playing the game more