If you hide a chart, Qlikview won't calculate the chart. So, at most you are displaying only one chart. So, if you are comfortable with show/hiding the chart it is ok to display all the charts in the same sheet. Even if you create 30 sheets with 2-3 charts in each one, it is very difficult to navigate between these sheets. You can show hide charts by using buttons grouped by functionality, I think this is better approach. If possible instead of 30 sheets you can minimize it to 3 or 4 sheets would be better.
Hope this helps you.