Since this is the top google search for me, I would like to clarify. This is not, as @AmanpreetSingh-MSFT suggests, the sum of objects this ACL will be set on due to inheritage.
It is the count of ACLs you are setting on the selected container, which will be inherited. So if you have 106 permissions and add 4, this warning will come to pass. Similarly, when you were to remove 4 ACLs, you would get the same warning about 102 permissions.
Another source states, that this warning only shows if 8 or more ACLs are set.