A helpful resource for hearing how English speakers pronounce things is Youglish. You can listen to its examples for enum to confirm that, as tchrist says, the first syllable is generally pronounced as /i/ (the same as the name of the letter E).

However, even if it is only used by a few speakers, it seems a pronunciation with an unstressed first syllable with a reduced vowel also exists (either /əˈnum/ or /ɪˈnum/), as demonstrated by the following links:

• Crockford on JavaScript - Level 7: ECMAScript 5: The New Parts (Douglas Crockford, at 26:03)

• Navigation: SafeArgs - MAD Skills (Chet Haase, at 7:40)

• Jeffrey Richter Using Reflection to implement enumerated types (Jeffrey Richter, at 15:33 and again at 15:54)

While you did not want information about the second syllable, others may be interested to know that it can either be /num/ or /nʌm/ (or /nəm/).

I don't think "Which one is correct?" is a very meaningful question, but in any case, if you use the pronunciation recommended by tchrist, you shouldn't have to worry about it.

A helpful resource for hearing how English speakers pronounce things is Youglish. You can listen to its examples for enum to confirm that, as tchrist says, the first syllable is generally pronounced as /i/ (the same as the name of the letter E).

However, even if it is only used by a few speakers, it seems a pronunciation where the first syllable is unstressed and has a reduced vowel (either /əˈnum/ or /ɪˈnum/) also exists, as demonstrated by the following links:

• Crockford on JavaScript - Level 7: ECMAScript 5: The New Parts (Nov 14, 2011; Douglas Crockford, at 26:03)

• Navigation: SafeArgs - MAD Skills (Oct 19, 2020; Chet Haase, at 7:40)

• Jeffrey Richter Using Reflection to implement enumerated types (Aug 21, 2018; Jeffrey Richter, at 15:33 and again at 15:54)

While you did not want information about the second syllable, others may be interested to know that it can either be /num/ or /nʌm/ (or /nəm/).

I don't think "Which one is correct?" is a very meaningful question in the context of words like this (compare the argument over "gif"), but in any case, if you use the pronunciation recommended by tchrist, you shouldn't have to worry about it.