There is absolutely no constraints regarding grid size or tile numbers. You can adjust these settings on the fly to be different from level to level or you can set a default value for all your levels.
The only thing to keep in mind when playing with the grid size, is that you may also need to adjust the scale of your grid and tiles to fit within your camera’s view. So let’s say you build a level with 7x7 grid and the next level uses a 9x9 grid. The latter level will need to be scaled to something like .5 or .8 to fit on the same screen size as the 7x7 level.
Alternatively, if you wanted to preserve the same tile scale for each level but also use different grid sizes, you could set a screen size that fits all the grid sizes that you plan to use.
Of course, all the settings mentioned above, are adjustable from the exposed values on the actor without any effort.
And to further clarify; In each level you will be placing a new M3_MainGame Blueprint which is the grid and play field. And all the settings within these Actors can be uniquely set for each of your levels.