Shouldnt there be a limit on how much tobacco, alcohol or over the counter medications someone can legally buy?

You make a great point, but you have to remember that with Prop 19, the objective is to get it legalized. Proponents will address other issues like quantities later. The process is incremental and will be worked on one step at a time, which allows people to adjust. Trying to pass legalization without some limitations would spell defeat for legalization efforts.