Originally Posted by tetracycloide

Adding additional warlocks beyond the first two will have drastically smaller impacts on ISB uptime even with extreamly high crit rates relative to the raid average.

For example:
2 affliction locks at 15% crit for SB casting SB 45% of the time (entry SSC/TK gear) will post a 47.7% ISB uptime.

Adding a destruction lock at 30% crit for SB cast BS 80% of the time (end game gear) will raise the ISB uptime to 62.5% for a total raid shadow addition of 2.96%

Adding a second destruction lock with the same stats will raise ISB uptime to 67.2% for a total raid shadow addition of 0.94%

Adding a third destruction lock with the same stats will raise ISB uptime to 69.5% for a total raid shadow addition of 0.46%

As you can see we're rapidly aproching the relm of negligable ISB uptime returns to addition destruction warlocks even with a huge gear differential in play. For the average raid, with everyone running with roughly the same gear level gear, after the second destruction lock there is almost no meaningful change in ISB uptime.