The goalkeeper, like any player, can be substituted at any stoppage of play, with the referee's permission. The important difference for goalkeepers is that they also need the referee's permission to switch positions with an outfield player (outfielders can normally interchange amongst themselves, but the 'keeper is a special case).
Play has to stop in order to bring on a substitute, whether it be a goalkeeper or an outfielder.
On a team there is only one goalkeeper on the pitch at one time. On a matchday squad there would usually be a substitute goalkeeper on the bench. At a football club there would usually be three goalkeepers or so.
Most indoor soccer games are played with six active players per team, one of which is the goalkeeper. Substitute players are permitted.Source: http://en.wikipedia.org/wiki/Indoor_soccer
Nothing requires a goalkeeper to have a maximum weight.
Casillas
The goalkeeper.
When a goalkeeper gets sent off, they have to leave the field of play and cannot be replaced by another goalkeeper. A teammate will have to step in as the substitute goalkeeper for the remainder of the match.
Gordon banks
julio Cesar (Brazil )
the goalkeeper
no :-x
To fool the goalkeeper.