a scroll bar will automatically appear, when contents of template grid contains more objects then client can show.
The number of objects displayed depends on the widget settings: https://docs.mendix.com/refguide/template-grid
Take a look at Number of rows/columns.
You might be able to tweak something like this with css. For example by setting the template grid to show 100 items.
Furthermore adding a css class where you set a fixed height and let the rest overflow in a scroll function like:
Overflow: scroll; }
A listview might however also be a good option to use.