Just as the Son of Man did not come to be served, but to serve, and to give His life a ransom for many.
1st Timothy 2:5-6 states: “For there is one God, and one mediator also between God and men, the man Christ Jesus, who gave Himself as a ransom for all, the testimony given at the proper time.”
John Calvin comments on Matthew 20:28: “‘Many’ is used, not for a definite number, but for a large number, in that He sets Himself over against all others. And this is its meaning also in Rom. 5:15, where Paul is not talking of a part of mankind but of the whole human race.” (Calvin’s New Testament Commentaries, A Harmony of the Gospels: Matthew, Mark and Luke, Vol. II, p.277, emphasis mine)
It doesn’t sound like John Calvin believed in a Limited Atonement. In your discussions with Calvinists, you may bring up Calvin’s commentary, but expect the Calvinist to insist that they are not responsible for defending John Calvin on all points, which is fair, but the point is that it’s not just Arminians that disagree with the perspective of a Limited Atonement.