The spelling of "duodecimal number" is fairly straightforward once you understand the IPA phonetic transcription. The word is pronounced /djuːəʊˈdɛsɪməl ˈnʌmbə/ which can be broken down phonetically as "dju-oh-des-ih-muhl" and "numb-uh". The "dju" sound represents the "d" and "u" in "duodecimal", while the "oh" sound replaces the "o" in "decimal". The "des-ih-muhl" syllables correspond to the remaining letters in "duodecimal", and "numb-uh" is simply the IPA transcription of "number".
The word "duodecimal" originated from the Latin words "duo" meaning "two" and "decem" meaning "ten". These words were combined to form "duodecim", meaning "twelve". Therefore, a "duodecimal number" refers to a numbering system based on twelve.