Figuring out a full series count always gets me tangled up between the main installments and those extra companion novellas. For the 'Royals' series, assuming we're talking about the contemporary romance ones by Erin Watt, there are three core novels that form the central story: 'Paper Princess', 'Broken Prince', and 'Twisted Palace'. That's the binge-able heart of it.
But then you've got 'Fallen Heir' and 'Cracked Kingdom', which follow a different set of characters in the same world. Some lists count those, pushing the total to five. And I think there are a couple of bonus short stories floating around digitally, like 'The Royals: Christmas', which just adds to the confusion.
Honestly, if you want a complete, satisfying arc without getting sidetracked, start with the original trilogy. Those three books have that addictive, soap-opera pull that makes you cancel plans. The later books are a spinoff, so the commitment feels different. My Kindle library says I read the trilogy over one very unproductive weekend.
Still, the exact number depends entirely on how deep you want to go into that universe. I'd say three for the main event, five if you get hooked on the world.