What would be Cat exam cut off for ABM stream for 2019-20?

There is no such stream wise cut off criteria for CAT. For being eligible for CAT, you need to have completed your Bachelor's degree with a minimum aggregate of 50% if you belong to General Category and its 45% if you belong to Reserved Categories. This applies for any discipline. But the rating of your Graduation scores in IIM Ahmedabad is as follows:

Percent score in Bachelor's Degree          Rating Score
<= 55                                                                          1
> 55 and <= 60                                                         2
> 60 and <= 62                                                         3
> 62 and <= 65                                                         5
> 65 and <= 70                                                         8
> 70                                                                           10

This rating changes for every IIM. This is just to give you a fair idea of the scoring.