Reserves is a plural noun, so 'too many' is the correct choice.
If it is a collective noun (always singular, no plural form) and you cannot count it, like rain, soup, courage, respect, laughter, funding etc. you'd always choose 'much.'
If it is a noun that can be counted (take a plural form) then you'd choose 'many.'
A strange thing about your example is, even though reserves is a collective noun that you can't count (collectively referring to money or resources), it is in the plural so requires 'many.'
(not a teacher, just a language lover)
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