What's the maximum number of bowlers that can be used to complete a over in test cricket? I mean if one bowler's injured then use the next bowler. And in recent history what is the maximum number of bowlers'd been used for a single over in a test match?
What's the maximum number of bowlers can be used to complete a over in test cricket
The only restriction in the Laws is that the same bowler may not bowl part of two consecutive overs (Law 17.8):
If for any reason a bowler is incapacitated while running up to deliver the first ball of an over, or is incapacitated or suspended during an over, the umpire shall call and signal Dead ball. Another bowler shall complete the over from the same end, provided that he/she does not bowl two overs consecutively, nor bowl parts of each of two consecutive overs, in that innings.
Therefore I think the answer to this one is ten - every player that did not bowl the previous over can bowl part of this over. However, it is obviously more than a little unlikely that this would ever happen as it would require bowlers to be incapacitated or suspended without bowling a legal delivery.
As an aside, I don't think the Laws contain any provision for what happens if ten players on a team are suspended from bowling, say for all running on the protected area of the pitch - but I strongly suspect the umpires would declare a forfeit if a team ever started behaving like that.