Yes, you can add or delete Rows by pressing your Up or Down Arrows, and you can add or delete Columns by pressing your Right or Left Arrows anytime during the Place Process until you let go of your mouse button. On as trackpad, I'm sure this is a lot harder to do.
An easier way would be to:
- Figure out ahead of time how big you want the outside dimension of your grid to be.
- Draw a Rectangle to those dimensions.
- Open your Scripts panel (Window menu > Utilities > Scripts).
- And in the Samples folder (which comes with InDesign) open the JavaScript folder and double-click on MakeGrid.jsx.
- In the Window that opens, put in your specs for how many Columns, Row, etc. and press OK
You'll get an instant Grid.
Select all of the rectangle in the grid and go to the Object menu > Fitting > Frame Fitting Options. Choose Fill Frame Proportionally (as a good starting point). And Press OK.
You have exactly the grid you want with images fitting those Frames nicely.
While Placing in a grid is a cool feature, I've never found it to be particularly useful for real work.