My best guess at a 53:
QB (3) T.Brady, J.Garoppolo, J.Brissett
RB (5) M.Gillislee, J.White, J.Develin, D.Lewis, R.Burkhead
WR (6) B.Cooks, C.Hogan, M.Mitchell, D.Amendola, A.Carr, M.Slater
TE (3) R.Gronkowski, D.Allen, J.O'Shaughnessy
C (1) D.Andrews
G (3) S.Mason, J.Thuney, T.Karras
T (4) N.Solder, M.Cannon, C.Fleming, C.Croston
DE (3) T.Flowers, G.Grissom, D.Wise
DT (4) A.Branch, M.Brown, V.Valentine, W.Hamilton
ILB(4) D.Hightower, D.Harris, H.Langi, E.Roberts
OLB(3) K.VanNoy, S.McClellin, M.Flowers
CB (5) M.Butler, S.Gilmore, E.Rowe, J.Jones, K.Moore
SS (4) P.Chung, N.Ebner, B.King, J.Richards
FS (2) D.McCourty, D.Harmon
ST (3) S.Gostkowski, R.Allen, J.Cardona
Total players: 53
I would guess that one of Hamilton, Moore, Richards or King gets bounced for someone else's DE cut.