According to LancsLive there are over 160 named championship players out of contract in the summer, with some having a 1 year extension.

Goalkeepers 22
Josh Vickers ( Millers one of them ), also Ex-Miller Lee Camp among the rest

Right backs 16
Olosunde aged 22 yrs, & Billy Jones aged 33 yrs, Miller players.

Left backs 10
Matt Penney aged 23 yrs at Sheffield Wednesday. ( Would have liked the Millers, to have gone in for him in the January transfer window )

Centre Backs 31
Millers players, Richard Wood aged 35 yrs, Clark Robertson aged 27 yrs, Jake Hull 19 yrs

Defensive midfielders 11

Central midfielders 24
Shaun MacDonald Millers player

Attacking midfielders 8

Left midfielder / winger 12

Right midfielder / winger 10

Forwards 21

Quite a few players will be looking for a club, then there is premier league players, who have reached their sell by date, looking for a championship club.
Looks like players will have to tighten their belts, & their agents getting a smaller fee.