Well, the word that follows after 'manche' will always be plural. That means the ending will be 'e' in nominative and accusative case, '-er' in genitive and '-en' in dative.
nom. Manche Jungen trinken Wasser.
gen. Das Wasser mancher Jungen ist kalt.
dat. Ich gebe Wasser manchen Jungen.
acc. Ich sehe manche Jungen.
According to Cambridge Dictionary, manch could also be translated "many" https://dictionary.cambridge.org/dictionary/german-english/manch
Ok so i just want to say something , you can translate this sentence as "some boys are drinking water" or "some boys drink water" both of which would mean very different things , would love to get input from a fluent speaker to understand how would one basically understand the phrases , is it just on context ?