Your search for a rule is admirable, but alas! doomed to failure.
- The plural of fish is fish. Unless you're differentiating between species:
The smaller fishes are more affected by ocean warming than the larger.
Or if you are a mafioso, in which case you say
Vinnie sleeps with the fishes.
Of if you're a theologian discussing the miracle of
the loaves and fishes.
Different species form plurals in unpredictable ways. Both tuna and salmon are their own plurals. Species that end in -ing, like the ling form their plurals by adding a final s, except for grayling and herring. You just have to look it up here. However, if the fish name is the name of a special at your restaurant, you might hear a waitress call out
I need two salmons and three tunas!
She means two orders of the salmon dish and three of the tuna dish.
No matter how many you have in a bowl, you only have fish, never fishes.
- Cakes is the plural of cake. It never means pieces of cake:
Some cakes have frosting; others have icing.
- The plural of fruit is fruits, but only when you're talking about different varieties:
Some fruits -- bananas, apples, kiwis -- are good for you. The rest are not.
You always eat some fruit.
- Drinks are the typical nonountable nouns, except when you're talking about varieties or individual servings:
Whiskeys are either blended or single-malt.
Give me two whiskeys, two scotches, two beers.
Waters has an additional plural as the naturally occurring water in a location, so during your vacation, you
take the waters at the spa at the hot springs
- Cheese follows the variety rule. If you have three cheeses on your cheese plate, then you have three different types of cheese, even when you have six pieces of cheese total. Same with milk:
I make three different nut milks in my blender -- walnut, hazelnut, and almond.
The plural of beef is beeves, but it's only used to describe individual animals, meat-on-the-hoof, so to speak.
The example "he died himself" is grammatical provided it occurs in a context which allows "himself" to be interpreted as a sort of intensifier. For instance, "The doctor devoted his life to preventing death, but in the end, he died himself." Or, the intensifier can occur immediately after the noun phrase it intensifies -- " ... he himself died."
At any rate, the "himself" in such a case is not the object of the verb, but serves another function.
(I'm not sure "intensifier" is a real grammatical term -- I just made it up.
Edit: Or else it was a dim memory. @Lawrence in the comments gives a reference to this nice typological account of intensifiers, which discusses the connection with reflexives: Focussed Assertion of Identity: A Typology of Intensifiers.)
Best Answer
Note: it might help you if you consider the word floor to be elided or left out of the sentence after ground. Saying floor twice is not necessary.
You can rewrite the sentence as
Floor in both sentences is a singular count noun. Hopefully you know that count nouns can be either singular or plural. This is one reason they are called count nouns. You can count them. As in
Compare:
I can elide or omit the first use of hand here. Saying it twice is not necessary.
In another article, the count noun floor is used in the plural:
So the answer to your last question is that you can say either "the ground and first floors" or "the ground and first floor". And you can say either" the ground and fifteenth floors" or "the ground and fifteenth floor".
In England, you walk into the ground floor and walk up one floor to get to the first floor. In the US, you walk into the first floor. (Exceptions can include when a building is built on a hill and on the higher side of the hill you walk into a floor higher than normal.)