J.E.
My daughter is also 20 months so I asked my friend, a dental hygenist, the same question. She said probably 3 or so as long as there were no problems and didn't notice anything while brushing her teeth (such as spotts on the teeth). One thing that helps brush my daugher's teeth is doing it in front of a mirror so she can watch it and also having her sing "awwww" and then brushing while her mouth is open. It is a game but we get it done!