Inventor
Continuously nerfed by Riot but still having many variants in the top strongest team compositions currently can only be the Inventor team. The common point of the popular Inventor compositions right now is that they stop at 5 champions to obtain the Tech Bear and will pivot to play with various other carries. Even a highly experienced player like T1 Bebe872, who has fought in the Korean Challenger rank, advises players to stop at 5 Inventors.
So why shouldn’t you play to the 7 champion threshold of this system? The first reason is that it is very difficult to achieve as you need the core that provides the Inventor’s Heart or Soul. Furthermore, the versatility of the Tech Dragon is also weaker than the 5 champion version as it buffs critical damage instead of direct attack power. Clearly, this is only suitable when you possess multiple Infinity Edges, so the Tech Bear at 5 champions is a much safer and more versatile choice.
Mage
6 Mages used to be an extremely powerful playstyle in the first half of Teamfight Tactics season 6 as it had all the elements to reach the top of the meta. At that time, this team comp had a strong burst damage champion in Lux, excellent healing and control capabilities from Janna and Yuumi, along with great tankiness from Vex. By pushing to the 6 Mage threshold, all of the aforementioned elements would be enhanced, making this composition extremely strong.
However, by the time season 6.5 rolled around, everything changed completely, and 6 Mages became an extremely weak playstyle. With Lux, Yuumi, and Janna being removed one after another, the Mage team lost its best offensive and defensive options. Meanwhile, the new carry named Ahri requires a strong frontline to cast her abilities multiple times, something that most Mage champions (except for Vex) cannot achieve. Therefore, at this point in time, you should only play up to 4 Mages and add some tank units for the best results.
Assassin
Perhaps no matter the meta, Assassins will always be an extremely “reliable” composition to use as the assassination capability of this system is always of high value. However, with many Assassin champions being removed in the mid-season patch, it has become extremely difficult for players to reach the 6 champion threshold of this system. Thus, many players at high ranks only stop at 4 Assassins when building their composition.
Moreover, even if you have the conditions such as possessing the Assassin Emblem or the Assassin’s Heart core, you still shouldn’t activate the 6 Assassins threshold. The reason is that the playstyle of this composition is usually rerolling and focusing items on a single carry. If you give the Assassin Emblem to another unit, you will only utilize its ability to jump to the backline, but the damage output will be minimal. Not to mention that you need a lot of Infinity Edges to maximize the effect of 6 Assassins, which is very difficult to achieve in the current meta.