There is no -s. To make a plural, you say the word twice, joined by a hyphen in writing.
buku ‘book’ becomes buku-buku ‘books’
anak ‘child’ becomes anak-anak ‘children’
But you usually do not need it
This is the part that matters. If the number is already clear, Indonesian does not mark it.
Saya beli tiga buku. ‘I bought three books.’
No doubling. The word tiga has already said three, so repeating the noun would be redundant, and Indonesian does not tolerate redundancy.
The same goes when the plural is obvious from context. Banyak orang ‘many people’ needs no doubling either.
So the honest rule is: double it only when you need to signal plurality and nothing else in the sentence does.
Doubling that is not plural
The same mechanism does other jobs, which is why you cannot treat doubling as a plural marker.
Jalan-jalan is not ‘roads’. Jalan is ‘to walk’, and doubled it means to wander about with no particular purpose. It is the standard answer to mau ke mana? and it means, roughly, nowhere in particular.
Pelan-pelan ‘slowly’ doubles an adjective to soften it.
Kadang-kadang ‘sometimes’.
How to tell them apart
Doubled nouns are usually plural. Doubled verbs are usually casual or repeated action. Doubled adjectives are usually softened.
You do not need to classify them consciously. Learn the common ones as vocabulary and the pattern will settle on its own.