Home of the Sovereign Protectors

My order was 19 or 20 yards total: 7 or 8 yards outer material and 12 yards inner robe material. And my seamstress said she'll have just a little outer and a good few yards of inner material left over. My shoulders are a lot wider than most people of my general height and build, so that added a bit of extra fabric for me that another person may not need to account for.

It's hard to estimate how much material would be needed via a forum post, without proper measurements and knowing more about the pattern being used. Most seamstresses will do measurements and, if they are comfortable, provide how much material they think they'd need to do the Royal Guard robes. I then added a yard extra onto each of them for my estimate, just to be on the safe side.

So it would be reasonable to order 12 yards for outer robe and 18 for inner robe ? I would have enough margin for a couple of small errors

That would likely be more than enough, for most body types. If you haven't worked with velvet, make sure you are careful. If you use a sewing machine the nap can cause the fabric to slide around a bit.

Also keep in mind if you order from outside Canada, you have to pay taxes/duty when it arrives. I believe it's around 10-12% on the order price, based on what I remember from my order.