about 5 or 6
One. Go to center line, wait for a stoppage of play, and request permission of the referee to substitute.
16-11(players including goalkeeper) + 5 substitute
just go to game plan and substitute players
At any stoppage of play, with the referee's permission.
Play has to stop in order to bring on a substitute, whether it be a goalkeeper or an outfielder.
Yes, there is no problem with this.
Yes the goalkeeper is counted as the substitute in football.
Ussually in a league game there are 6 or 7 substitute players but only 3 of them may enter the pitch during a league game. In a friendly there can be a maximum of 12 substitute players and the number of substitues a coach can make is unlimited.