What Length are the M5 pronged tee nuts for spoil board

I have done the drop in m5tee drop in tee nuts and i am at the stage of adding all the cuts for the pronged M5 tee nuts what length are they as they come in different sizes and how many do I need. I am using 18mm board. Also what free software can i use as i did not realise it cost so much.

Thanks E