If it’s not one, it’s a crowd. This is, in a nutshell, the basic rule to distinguish between singular and plural in English and most other Western languages. In Japanese the situation is a little different, though, as grammatical marking of plural is optional and — as a result — organized quite a lot less systematically.
The form that finds itself most commonly entrusted with the expression of plurality is the suffix -tachi. It turns 子供 (kodomo, child) into 子供たち (kodomo-tachi, children), 男 (otoko, man) into 男たち (otoko-tachi, men) and 私 (watashi, I) into 私たち (watashi-tachi, we). While this looks fairly straightforward — in fact, it’s the English plural forms that are irregular in these three examples — the situation is way more complex.
First off, -tachi normally can be used only with animate referents, ideally human. While ペットたち (petto-tachi, pets) is still acceptable, particularly in this pet-loving society, 電車たち (densha-tachi, trains) is rather weird unless you are talking about Thomas the Tank Engine and his friends. A second point is that -tachi does not faithfully multiply the singular form, but often refers to the group around the tachi person. Thus, when I have a drink with Tarō-tachi, that does not mean I’m going out with many Taros, obviously, but with Taro and his gang. Also, -tachi can be quite inappropriate for persons of higher status. A frequently quoted example is the plural of 先生 (sensei, teacher/doctor/professor), which rather than sensei-tachi, should be 先生方 (sensei-gata).
Another common plural marker is -ra, the usage of which, again, is largely restricted to human subjects. The form is not overly respectful, as can be seen from the fact that it occurs in casual or derogatory expressions such as あいつら (aitsu-ra, these guys) or やつら (yatsu-ra, those bastards). On the other hand, maybe because of its brevity, it is quite often used in news reporting. For instance, the group that won last year’s Nobel Prize in Medicine was frequently referred to as 大村さんら (Ōmura-san-ra, Mr. Omura and his fellow awardees) in the Japanese media.
An alternative way to express plurality is duplication. Quite unfortunately, this amazing sleight of hand is restricted to only a handful of words, including 我々 (wareware, we), 人々 (hitobito, people) and 国々 (kuniguni, countries). Rather than mere plurality, duplicated forms frequently imply some heterogeneity of the things described. This may explain why there is 山々 (yamayama, mountains) but no 川々 (kawakawa, which would mean “rivers”): At least in Japan, mountains are considered more variegated than rivers. Likewise, duplicated time expressions like 年々 (nennen) or 月々 (tsukizuki) do not predominantly express a larger amount of time, but emphasize the individuality of each single item. That’s why these expressions are normally translated as “each year” or “per month”.
In Japanese, plurality can also be expressed through prefixes. Though again somewhat restricted in application, 諸 (sho) does a good job here. It gives us 諸外国 (sho-gaikoku, foreign/other countries), 諸問題 (sho-mondai, various problems) and 諸経費 (sho-keihi, expenses), among others. A second useful plural prefix is 数 (sū), which best translates as “several,” as in 数世代 (sū-sedai, several generations) or 数名 (sū-mei, several people). Particularly favorable vocabulary types for sū are units, like in 数時間 (sū-jikan, several hours) or 数ページ (sū-pēji, several pages).
Finally, we shouldn’t forget ス (su) and ズ (zu), the Japanized versions of the English (voiceless and voiced) plural “s.” Though most loan words tend to get stripped of their plural on entering the country — such as サングラス (sangurasu, sunglasses), ロック (rokku, on the rocks) and レディーファースト (rediifāsuto, ladies first) — there are also cases where the suffix is retained, someway or other.
An example only for adults is the bestselling erotica novel “Fifty Shades of Grey,” which came to Japan as フィフティ・シェイズ・オブ・グレイ (Fifuti Sheizu obu Gurei). As for minors, there is トイザラス (Toizarasu, Toys R Us). An English-like plural is also almost obligatory for names of baseball teams, from 阪神 タイガース (Hanshin Taigāsu, Hanshin Tigers) to 中日ドラゴンズ (Chūnichi Doragonzu, Chunichi Dragons).
In a few exceptional cases, even non-loan words can take an English plural. One example is the male subsection of my local PTA, which call themselves 父ちゃんズ (tōchanzu, “daddies”). Another case in point is the 2014 Japanese comedy film 女子—ズ (Joshiizu), which multiplies the term 女子 (joshi, woman) using the English plural suffix.
As these examples show, Japanese speakers can go some way to overcome the “singularities” of their language when talking about things and people that happen to be more than one.