Crusader Kings III

awesome, does that mean, for example is you play with the byzantines that automatically start with 10 kingdoms, if my ruler dies and i have 10 children will automatically be inheritated without even creating the kingdom titles? If yes thats an easy case indeed !

Sorry for the late reply.

No!

Empires will keep kingdoms TOGETHER. You must have (potential)-Kingdom titles without an Empire! Very very important!

HARD DISAGREE! DO CREATE AN EMPIRE! You can grant independance to vassals (an option among the vassal interaction menu, below grant or retract titles) and as an emperor you can have kings as vassals. It is much easier to first create a powerful empire that won't immediately fall apart in case your ruler should die. Once you have at least 10 vassal kindoms in your empire, make sure every one of them is held by a member of your dynasty (doesn't have to be a child, any member of your dynastie will do!), then grant them all independance.

This is much easier than having ten or more living children (yet alone ten sons, if you don't play a religion or culture that enables equal inheritance) and conquering all of those 10 kingdoms within the lifetime of your first character. If your character dies before you have both "tens" full, all of your progress can be lost if you are unlucky, because there is no guarantee that those of your NPC children who inherited a kingdom from your first character will all manage to keep that kingdom title (respectively to keep it in the family) until your second character conquers some more kingdoms to inherit. Some will be dethroned by rebellions, some may be conquered by foreign NPCs, some might fight each other, the girls might get into patrilineal marriages with characters from other dynasties ... there is much random chance involved once you fail to assemble ten heirs and ten kingdoms in the first attempt and all it takes you to fail is an untimely death by one of the many hazards of the game. With an empire, the death of your character will only be a minor drawback, as long as you have a well established heir to keep the empire together and to keep expanding it.

If you want to combine this with other difficult long-term achievements, playing in Spain to get either the Reconquista achievement or the Al-Andalus achievement is a good option, because after you have conquered all of Iberia (plus Aquitaine for the Al-Andalus achievement as a Muslim ruler), you will already have 8 respectively 9 Kingdoms and just have to conquer 1-2 more before you can devide your empire among your family and get the nepotism achievement as well. Uniting Africa is of course also an achievement that allows you to get the nepotism achievement afterwards as a bonus, if you are doing that one (I did it, but the second half of that playthrough wasn't fun, because once you have a big stable empire and have advanced your war technology way beyond your tribal society neighbors, conquering all those other countries becomes mostly a boring longwinded chore with no enemy even remotely strong enough to challenge you, unless you get unlucky and get targetted by a great holy war).

EDIT:
Unfortunately, it isn't possible to just grant indepence to a Kingdom that is de jure part of your empire, so it isn't quite as easy as I suggested in the previous paragraph. You must either conquer 9 kingdoms that aren't de jure part of your empire or you must get a second empire title, make the new one your primary title and then destroy your prior empire title to be able to just grant independence to the kingdoms of your former empire.
Alternatively, you can try to provoke your vassals into creating a dissolution faction and give in to their claim to dissolve your empire into independent kingdoms. Still easier and more secure than having tribal partition law and just hoping you manage to conquer ten kingdoms and have ten eligible heirs before your character dies.

A possibly superior alternative might be to have at least the second stage of inheritance laws (in which new title's AREN'T automatically created to give them to additional heirs) and conquer ten kingdoms without creating an empire, but also without creating (respectively destroying) the additional kingdom titles until you have ten and are ready to part your realm. That way, your realm wouldn't automatically fall apart if your ruler dies prematurely.