Learn English – Is the plural “bosoms” an acceptable word? Or is it always “bosom”

In English it's usually an uncountable (mass) noun, with "piece of spaghetti" or "strand of spaghetti" used for the "singular", but in Italian spaghetti is plural (of "spaghetto"), and they freely use it as such. (See this website.) All the citations in the OED for "spaghetti" are consistent with its being used as a non-count noun, so I don't think Maugham's "his spaghetti were…" was ever normal usage.

Your example sentence is fine. The plural of fruit is fruits. You are confused over the matter of countable and uncountable nouns.

This is tricky to explain, because there are few strict rules about which nouns are countable and uncountable, so I will hope you will forgive this over-simplified account:

• Some nouns (e.g. chair) are countable. We can say "one chair", "two chairs", etc. They have singular and plural forms.
• Other nouns are uncountable. We do not say "one fun" "two funs". There is no plural form of fun.
• Many nouns have both countable and uncountable senses. E.g. you can have a bar of chocolate [uncountable], or a box of chocolates [countable, plural].
• Nouns for classes of foodstuff (fruit, meat, cheese, etc.) are usually uncountable, but they take a countable sense when we talk about different varieties (a wide selection of cold meats and cheeses).

Your example sentence talks about different varieties of fruit, so fruits is fine.