3 - Russell Martin, Wes Hoolahan & Grant Holt. Few others were playing at that level but with other clubs (Bennett, Crofts, Johnson, Pilkington, Morison etc..). Very few youth graduates, if any, involved though.
Ankergren & Navarro have both played in the Championship as well. Not often enough to count as "experience" though.
As a promoted L1 side, it takes time to build a Championship squad. It's not really something you can do in one summer, more of a gradual process - identifying those who are not...