Becoming a knight of a noble family in the world of Atharia is very rare and one of the greatest honors a commoner can be granted by the noble family they serve. This is granted to commoners who have been notable with their physical prowess, To the noble family they serve, or are a serf of, they have shown their skill and drive to protect their lands, their family, the nobles, or just the world from those intended to do harm/ Family Knights are quite rare and limited. They serve a noble house that has lands. Each landed noble house has only one knight and do not remove a knight unless they are quite a scandal in their actions.
Scandalous actions are things like betraying their noble family, committing treasonous acts. Basically, if it goes against the noble family in some form that makes them look bad a knight is put to death. Death is the only way knights can be removed. No other noble will try granting them quarter because a knight taking scandalous actions makes the knight considered untrustworthy and creates concern for them protecting the family they serve. Any squires a scandalized knight has are also put to death. As they learned from the knight and the risk of them carrying on what their knight has done is considered to great. The only exceptions to a squire being put to death is if they are still quite young or they have not being training with the knight for long.
On the note of squires, knights can have, up to 3, squires. Only one will be able to inherit the title of knight from the knight they serve so a knight taking on multiple squires can create competition between the squires. Squires are generally taken on when they are between 6 and 10 years old, much older than that and it is often considered they can not get the proper training. It is not uncommon for an older knight to take on training the squire of a younger knight, though, that squire is considered the older knight's until the younger knight feels comfortable and is well equipped to handle the training of their squire(s).
There are plenty of knights that are not knights for families. They are not held in as high of esteem as one that has been selected to be the official knight of a noble family. They have been knighted for the deeds they have done and the actions they have taken.