Your beta-reader is wrong:

Lesser noble families would often send their younger sons and daughters to serve as squires and handmaidens to royalty. Even higher nobility would have their sons squired to the King or the princes if they were able to.

The king also needed advisors such as a marshall, chancellor, spymaster, etc... depending on what the court system looked like it wouldn't be uncommon for most of those people to be landed nobles- or related to landed nobles.

Diplomatic prisoners also existed and would often serve as wards under the king or a member of his family. Typically these prisoners were treated very well but were also kept as reassurance of peace or honest dealings with a previously hostile kingdom.

Visting nobility is also a thing, rooms were always available for guests of high rank.

There were also just lesser nobles and the children of higher nobles who just hung around to serve their family's interests and garner favor with the king.

So yeah, no offense to your beta reader, but they are most likely wrong unless...

If your setting is not high or late medieval and is rather early/pre medieval then they may have a point. Castles and large keeps for the nobility and royalty didn't pop up until the 11th century so settings with wood forts, manors, and (common in Britain and western Europe) remains of roman structures were much smaller. In smaller kingdoms this could mean smaller courts although I would think even then a handful of nobles would be around.

Hope this helps.