Steffi won 11 Slams since the stabbing of Monica Seles.
Take away the ones she had not been able to win with Seles competing before having her career devastated by the knife of one of Graf's fans and you only have 3 since Seles was stabbed. That would leave Steffi with 14.
Monica Seles was never the player after her return that she was before the stabbing ... and anybody who has paid attention to tennis in these last 15 years would know that.
The only Slam that Graf had a good chance of winning with Seles in the form she was in during those years 1991, 1992 and 1993 before the stabbing was Wimbledon .. so, a good argument could be made that Graf at her best and Seles at her best throughout the 1990s would have brought Graf only a total of 14 Slam -- the 9 Slams Graf won before Seles' dominance began + the 5 Wimbledons Graf won after Seles' dominance began.