Go either 2x10 or 3x5, depending on what you want them to do. Three small units lets you snipe backline heroes, Stormcast ballistae, that lone objective being kept by a unit of five Revenants, while 2x10 gives you two solid smash units that can target more or less anything. Given your setup using Shroudguard and only 20 Grims, I think 2x10 is the best choice. 15 or 20 rarely pays off until you've lost nearly half your unit, at which point you're already in trouble.
As for Darrakar, you absolutely should. So underrated, especially if you run Chainguard.