Why is the plural not formed with a "-t" here?
When the noun is preceded by a numeral, then it changes to partitive case. Kissat -> kaksi kissaa.
It's partitive singular. If the number is in the nominative case and is higher than one, its noun is in the partitive singular.
It's a bit like saying "two of cat", "three of cat" ...
Is that for all numbers higher than one? How about 21, 31, etc?
It's for all numbers except one (including lower than one). This is because in these cases in Finnish yksi behaves like a pronoun, not a numeral.
We need more cats.