I have about 50 max empty space for handling this event's inventory demands.
You need 35 space to handle the 17 MetalGarurumon, 17 WarGreymon, and Gift Omegamon. Then an extra 6 spaces to handle a frag fusion at any given time.
I am sure you can manipulate your inventory further than that, but I do but want to over-farm. I want enough time for Vademon after all is said and done.