The crownlands and the iron islands are not kingdoms.
The name seven kingdoms originates from the seven kingdoms, which existed before Aegon's conquest. These are:
- Kingdom of the North (Stark)
- Kingdom of Mountain and Vale (Arryn)
- Kingdom of Isles and Rivers (Hoare)
- Kingdom of Rock (Lannister)
- Kingdom of the Reach (Gardener)
- Kingdom of Storm (Durrandon)
- Principality of Dorne (Martell)
The iron islands were part of the Kingdom of Isles and rivers, House Hoare hailed from there and conquered the riverlands a century or so before Aegon's conquest from the Storm Kingdom. House Hoare was virtually destroyed in the conquest. Aegon divided the Kingdom of Isles and Rivers into the Iron Islands and the Riverlands and gave two local houses Greyjoy (selected by the ironborn themselves) and Tully (the first house to rebel against Hoare) control as his Lords Paramount.
What is now the crownlands was a constant warzone between the Reach, the Rivers and the Stormlands vying for control over this area, as such it was a prime spot for establishing a beachhead in Westeros. After the conquest, Aegon declared this land Crownlands and founded the capital there.
The name seven kingdoms however still originates from those seven kingdoms, which were there before Aegon, he did not include the two constituencies he created in that. The North was one of those original seven kingdoms, since it's independent now, by all rights, it's now the six kingdoms.
Note that the "Seven Kingdoms" were already only six for the most part of their history. Aegon still called himself Lord of the Seven Kingdoms, as he always laid claim to Dorne. But Dorne didn't come to the fold until 200 years after the conquest, so only 100 years before the show, after marrying into House Targaryen. Which makes it also a questionable choice of the showrunners for Dorne not to demand their independence as well.