I do think there should be a maximum defense pact, dependent on total number of remaining civilizations. It is just ridiculous to have 7 total players, and all the AI have 4 or 5 simultaneous defensive pacts.
"realistically" as much as it is possible within a highly unrealistic game, these pacts should be determined by military strength. The weakest civs should be trying to cozy up to the strongest in hopes of delaying being conquered, at least as long as said strong civ has been show to be "reliable". Also, Civs that are not the strongest should be banding together in defensive pacts, in hopes of fending off the strongest. These competing strategies should make the game more interesting, but right now, even with 6-19-2, they seem to mostly follow one variant of a single strategy, and it really cuts the fun level for the human player.