It is possible (if a little unlikely), to start with 20 in a stat. If you roll for stats, doing the standard 4D6 and dropping the lowest value, you can get a maximum roll of 18. Combine this with taking a +2 from racial abilities, and you can start off with a maximum of 20 in that stat.
I’d advise checking with your DM before doing this, as it can lead to an imbalance of power between players and also removes any chance of character growth. For me, some of the enjoyment of DnD is watching your character become more powerful as you progress, which can be taken away by starting with a 20, especially if it’s your primary stat.