Technically, it should be repetition: stroberi-stroberi.
But we rarely use plural form in daily conversations; whether you eat only one or eat a lot of strawberries, Indonesians just don't care. It's irrelevant for us. As you can see, Indonesians don't really pay attention to details.
So, when you eat 30 strawberries, just say: "Saya makan stroberi". That's enough information for us
suggested meaning of "manis" is "sweet, sweetened", therefore, a possible translation is "I eat sweetened strawberries"
Bahasa Indonesia uses a different phrase for sweetened stuff, "manisan".
"manis" (adjective) = sweet.
"manisan" (noun) = sweetened stuff.
(adjective) + (suffix -an) = noun.
Like this :
"Saya makan stroberi manis." = I eat sweet strawberries
"Saya makan manisan stroberi." = I eat sweetened strawberries