How many orbs should I save for the future Brave Heroes banner?

As MUCH as you can muster.

You're at the mercy of the gacha so there is always a chance that all of the 5 stars you summon may not be any of the new brave heroes.

Saving up and hoping for the best (while being prepared for the worst) is the most reasonable advice you can receive.

If you want the Brave Heroes, I would say save as much as you can. From getting lucky by getting one on free summon until getting unlucky with pity brakes and no focus unit after +200 Orbs, everything is possible.

If you are only interested in one of the brave heroes, you don't need orbs because you can probably choose one with neutral IVs, if IS isn't changing anything there and now suddenly do the same thing like they did with the anniversary banner.

1,000!

Broken record here again to tell you, really, as many as humanly possible. You will need it.

I had 200 and got lucky with all 4, plus a free pull.

Phoenix master had 400 and got crap.

Point is, the more the better but anything goes.