Recommened nailing schedue for cripple studs between header and top plate
If you can nail down through the top plate (e.g. before the second top plate is installed) then 3 12d nails is standard for 2x6 framing. For nailing the cripple into the header, toenailing should be fine, but I'd use 12d glue coated framing nails (greenish nails). Save the 8d nails for attaching sheathing.

Note, this is just personal experience, your local codes may require otherwise. It doesn't hurt to ask your building inspector what they require.