Lots! But all chickens will fly at some point, I find my heavier and more 'fatbutt' girls like my cochins and welsummers aren't much of flyers compared to my booted bantam or barnevelders.
Pure Silkies cannot fly as their feathers lack the barbicels which fuse the feathers together and enable them to catch air giving them the lift necessary for flight (although they are great jumpers). Unfortunately though a lot of hatchery quality Silkies do have barbicels in their feather so there is enough fusion in the flight feathers to get some lift. It's a major breeding flaw in these hatchery quality Silkies.