Editor script multiple levels of toolbar

I’ve created an editor script that shows a toolbar of different buttons along the top, the view changes depending on which toolbar button is selected. I’m currently using this line:

toolIndex = GUILayout.Toolbar (toolIndex, parsedTools, EditorStyles.toolbarButton );

but there are too many options that scroll off screen. How can I set it so it shows 10 buttons then shows the next 10 buttons underneath those, rather than all in one single line?

Used the SelectionGrid instead.