Last night I ran a session of Blades for 9 players, and it went surprisingly well.
Four of the people at the table had played before; the other five were novices. The score was the artifact one from War in Crow’s Foot (the PCs had to place the weird artifact in the Red Sashes’ sword academy).
The party wound up splitting up, so I had to bounce around a bit between the different groups, but we kept the forward momentum going really well and everyone had fun. A group that big is obviously not ideal, but it’s workable.
One thing we noted was that, if you have a large group and multiple players use their downtime actions on the same project, they can create and resolve long-term projects in downtime really fast. Just something to be aware of.