Wenn es denn Entwickler sind, würde ich auch 'a team of 50 developers' sagen.

Man kann sagen, 'a staff of 50', aber nicht, 'a team of 50 staff'.

Du brauchst auch gar keinen Zusatz: 'a team of 50' geht auch.

It may depend on what you mean. Do you mean simply that the team consists of 50 individuals? If so, you could say a team of fifty or fifty people (or developers). Do you mean that the team is drawn from the company's staff? Then you might say a team of fifty staff members or fifty employees.