Consisting of National Mode and Club Mode, the Bonding System opens at Lv.25, in which two or more on-field players from the same country or club will automatically bond together. The Bonding buff/bonus will take effect by using corresponding items.
The max buff/bonus from both modes is 3025.
For Club Mode, the buff / bonus is: 400 (combo of 2) ->825 (combo of 3)->1100 (combo of 4)->1255 (combo of 5)->1650 (combo of 6)->1925 (combo of 7)->2200 (combo of 8)->2475 (combo of 9)->2750 (combo of 10)->3025 (combo of 11). Do the maths and you can find that the best Bonding combos in Club Mode are: 4-4-3, 8-3, 7-4, and all 11. We can build a team with 4 Real Madrid players, 4 Bayern Munich players and 3 Barcelona players to achieve the max buff/bonus. Please note that since CL players can choose any clubs they played for, they are more flexible in Bonding. For instance, Beckham can be a Real Madrid, Man. Utd, AC Milan, or PSG player. The Transfer Card is available in the shop, but once you use it, this CL player's Bonding points will become 0. You need to re-activate.
For National Mode, the buff / bonus is: 370 (combo of 2) ->885 (combo of 3)->960 (combo of 4)->1255 (combo of 5)->1770 (combo of 6)->1820 (combo of 7)->2000 (combo of 8)->2655 (combo of 9)->2750 (combo of 10)->3025 (combo of 11). Hence, the best Bonding combos in National Mode are: 3-3-3-2, 9-2, 6-5, and all 11. Given players' nationalities are fixed, you cannot change them in the game.