A shitload...
Depends what wood your using. Double up each side if using mdf, get some allthread that goes top to bottom, depth wise, and width wise if you can.
If using birch, just double baffle and then do as I said for the rest of mdf
And I'd lay a little bit of glass inside too, atleast a few layers in all corners. Too much bracing never hurts, if ou have the space