Just how many dukes are there? They are dukes/crown prince of different countries right?

so regarding the seconds knights I can tell you that there are also first and third etc knights. As you read f.e. in this last chapter when they met the uke asked the crown prince for cooperation from the first knights. It was also explained the beginning of the story that the first knights are right under the cron prince and to protect the royal family whereas the second knights are located a bit farther from the castle to do more like "city work" and the third knights are even farther out etc. So it's like a rank system and first knights are like the musketeers and second knights come right behind that. Regarding the different princess I believe they are in some way family. I believe it is a similar system to the ones in like the 18th century and before where royal families married between themselves to strengthen ties (incest etc.) so I believe the duke could actually be like a distant cousin or something from the crown prince and so on. But there is only 1 crown prince and therefore only 1 successor to the throne the others are just extras lol. but being a duke does not mean that they have to be a prince and related to the crown prince. They can simply be super wealthy and have passed down the title through generations, it could simply be a social status and not like the crown prince title which is determined by blood and the "celestial will" whatever...

OH MY GOD I appreciate this soooo muchTHANK YOU VERY MUCH..
i thought duke, king , emperor were same, from the novels I've been reading so far...
And about the knights, it was so obvious yet I couldn't pinpoint exactly. Like MC and his group doing all detective and police works A.K.A civil works like you said hajajajahaha I'm crying I feel so dumb

all good haha I know it can be confusing so I'm happy that I could be of help

Thanks again (✿^‿^)