Written, comfortable has four syllables: com-for-ta-ble. In everyday speech most people say three, because the weak middle syllable drops out. Said carefully it has four, and that is the form Cambridge lists first.
The dictionaries differ. Oxford gives only the short form, KUMF-tuh-bl, for both accents. Cambridge lists the long form first. Either way, the stress stays on the first syllable.
The first syllable trips people up too. Com sounds like come, with the vowel of cup, as in other words where o sounds like u: some, money, mother.
English drops weak middle syllables in many everyday words, such as vegetable, chocolate and different. Use either length, but keep the end light: tuh-bl, never table.