A preceding plural word is used for all words to indicate plural, and words referring people(not only humans beings since the language was not originally spoken by humans beings of that world.) also uses reduplication to indicate plural.

for words referring people, reduplication is more common.

The marker for the plural is "i". It is a suffix, unless the word contains a prefix; in this case, the "i" will move to the prefix: a "neutral" prefix is usually comprised of a consonant or a consonants cluster + the "neutral" vowel /e/. But when the word is plural, the "e" drops and is replaced by the "i".

The "i" itself is part of a system of marker vowels wich indicate number and, optionally, gender : a(feminine), u(masculine), i(plural). The vowel "e" is neutral in the sense it is a default vowel, which doesn't represent any gender or number.

When there are two adjacent vowel markers (possiblities are a+i and u+i), the gender marker is placed before the number marker when they are in a suffix position, and after when they are in a prefix position; in both cases, they diphtong and become -aj, -uj, -ja or -ju.

Some examples:

PAN, a person, someone; PANI: some people, PANU: a man, PANAJ: some women.
KEBAN, a child (literally, a "little person/human": ke- is a diminutive prefix–note the /p/ of "pan" gets voiced); KIBAN: some children, KABAN: a little girl, KJUBAN: some little boys.

Affixes can be added one to each other: as "le-" is a determining prefix (which plays the role of determined article); "LEBAN" will be "the person, this person" and "LEGEBAN" will be "the child, this child"; in this case, only one suffix will be modified by vowel markers if needed, the first one. Note, that a determining prefix will always come before a diminutive prefix.
So "the children, these children" will be "LIGEBAN".

It is a matter of interpretation to determine if those affixes are polysemic or not. Indeed, we can see the -e- as euphonic, serving only to link the consonantal prefix to the base and thus its dropping would only be the lack of need for it to be, as another prefix (i, a, u, ja, ju) follows the consonantal prefix. This way, the diminutive prefix is k-, and a gender or number prefix can be added to the base too, so "ka" would be prefix 'k' + prefix 'a'. Else, if there is no gender or number prefix, a euphonic /e/ is inserted.
But we can also consider a 'composed' prefix, like "li", to be a declension of the basic-neutral prefix "le", as -i marks the plural. Affixes would thus be declined in gender and number.