101 runs

To answer this question, the information provided in the question is insufficient. One crucial piece of missing information is in how may past matches he had got out.
Assuming that he had got out in all the 50 matches already played: If he goes on to get out in the next match, he has to score 101 runs in that match to get his average to 51. BUT if he remains Not Out in that match, he needs to score 50 runs only to get average of 51.

This is because of the peculiar way in which batting average is calculated in cricket. The average is calculated as (Total runs scored / Total innings completed), where “completed” means completed by getting out. If the batter remains not out in any innings, number of such innings are NOT counted for calculating average, even though the runs scored ARE considered. Thus, if a batter scores, say, 60 runs in one match and 40 in the next match and gets out in both matches, his average will be 50 ((60 + 40) / 2) whereas if he remained not out in one of the matches, his average would be 100 ((60 + 40) / 1).

Since John’s average is 50 after 50 matches (under the assumption of having got out in all the matches), the total runs he had scored in those matches must be 2500. If he were to get out in the next match with average of 51, the total number of runs must be 2601. (51 runs x 51 completed innings), which means he has to score 101 runs in the match to reach the target. However, if he could remain not out in the match, No. of completed innings would remain 50. So, to achieve the average of 51, the total runs has to be only 2550 (51 runs x 50 innings), which means he needs to score only 50 runs in that match.