Learn English – Are there any words in English that have a plural with a separate derivation

This is all down to the fact that English is a language of acutely mongrel lineage. It has substantive roots in Celtic, Romance and Germanic languages (to name a few) and a grammar that lends itself well to the adoption of "loan words" (non-native words adopted into the native tongue.)

The "standard" means of pluralising a noun is to append -s, with some conventional variations (eg -f becomes -ves, -y becomes -ies) for convenience in spelling and pronunciation.

However latin-based words tend to pluralise in the latin fashion, so for example bacterium becomes bacteria, and cactus becomes cacti. Similarly greek-based words will adopt the equivalent pluralisation appropriate for the original root.

Still other words of Saxon or earlier origin have lovely, earthy plurals that defy the "conventions" due to their traditional forms being maintained. Geese, Mice and Children owe their unusual conjugations to their ancient roots, and to the fact that they are common words whose everyday repetition keeps them from slipping into bland conformity.

In my experience, words which do not pluralise are those which relate to herding, hunting and the counting of animals. These words tend to be saxon (germanic) or celtic in origin owing to the presence of farming and hunting in Britain long before the Norman invasion. This can be inferred by the fact that sheep, cattle and game do not pluralise, while whales, sparrows and elephants (seldom hunted or farmed in Britain!) definitely do.

I suspect these tend to be a contraction of the traditional counting forms for such cases ("head" of cattle, "brace" of partridge, "shoal" of fish) but this doesn't really answer the question of why such plurals take the same form as the singular. It could be that when counted in such a way, the animals being counted were considered an uncountable, continuous quantity (similar to water or money) that could only be "counted" when quantified with their associated counter, so cattle would be rendered an uncountable noun by its quantifying counter head. It's interesting however to note that bird pluralises to birds, while aircraft does not pluralise.

Sadly for the non-native speaker, this makes learning the "rules" of English an arbitrary and frustrating affair. However, spare a thought for the Japanese, who do not have plurals for any but a few unique nouns, and must instead learn a separate counting-suffix and corresponding character (kanji) for almost every class of noun imagineable. There are in fact entire volumes of the things, and it would be nigh-on impossible for any person to learn them all. Wikipedia lists a choice selection.

The answer appears to be no, there aren't any such plurals. The comments section on the question have pointed out a few interesting cases but they don't apply to the specific requirements you were looking for:

numerical counts: one car => two hundred cars

article removal: a car => cars

collective nouns: a sheep => a flock of sheep

One additional exception would be adjectives that become invalidated when there are two:

a single wolf => two single wolves

But since your restriction appears to be of the form [0-9] [noun] I think you can escape these.

The only evidence I have to offer on my behalf is Wikipedia's article on plurals and their extensive irregularities section. Nothing I saw on the article had an example of what you were looking for.

If anyone happens to come across an example I am more than willing to update the answer. In the meantime, I think it is safe to say that there are no such plurals -- and if there are they are extremely rare cases.