How many peaches in 桃是水果? How do we know if there is only one peach or multiple peaches in the sentence
桃是水果?
How do we know in general? With the use of measure words?
https://en.wikipedia.org/wiki/Chinese_grammar

Plurals
Chinese nouns and other parts of speech are not generally marked for number, meaning that plural forms are mostly the same as the singular. However, there is a plural marker 们 [們] men, which has limited usage. It is used with personal pronouns, as in 我们 [我們] wǒmen, meaning "we" or "us" (from 我 wǒ, "I, me"), and can be used with nouns representing humans, most commonly those with two syllables, like in 朋友们 [朋友們] péngyoumén "friends" (from 朋友 péngyou "friend"). Its use in such cases is optional.[13] It is never used when the noun has indefinite reference, or when it is qualified by a numeral.[14]
The demonstrative pronouns 这 [這] zhè "this" and 那 nà "that" may be optionally pluralized by the addition of 些 xiē, making 这些 [這些] zhèxiē "these" and 那些 nàxiē "those".

Since Chinese nouns usually do not have plural form, we have to use plural markers to indicate plural like the Wiki article stated, you can also indicate a noun is singular or plural with measure words like 一個(a /one), 兩個 (two); or predeterminer like 所有(all) 有些(some)

In the case of " 桃是水果" we can deduct from the context that we are talking about "all peaches" , because 一個桃是水果 (a peach is fruit) doesn't make sense.

In 桃都是水果, 所有 is implied