the only correct answer is "Whatever you're not using"
There's a few decisions that you'll need to make along the way to determine what you can fill.
The first few I can think of when looking at this picture:
Are you using a manual or electronic throttle?
Do you want the trim tag on the firewall?
Are you using a stock style or aftermarket wiring harness?
The best way of going about it is determining what the holes are for and if you'll need it. If you find a Factory Assembly manual you can quickly determine what they are.