Diphthongs and How Do Linguists Count Vowel Phonemes?

How do linguists decide whether a diphthong is a distinct phoneme of just two vowels next to each other in a given language? For example, I've heard that English has up to 20 vowel phonemes*, which include diphthongs, while Italian has 7 vowel phonemes*. However Italian also has diphthongs and if we include them, then Italian has more than 7 vowels. The /ɛi̯/ sound in as in "lei" is similar to the English FACE vowel, the /ai̯/ sound as in "hai" is similar to the English PRICE vowel, the /oi̯/ sound as in "noi" is similar to the English CHOICE vowel, the /au̯/ sound as in "auto" is similar to the English MOUTH vowel and the /ui̯/ sound as in "lui" or the /u̯ɔ/ sound as in "può" don't have equivalents in English.

*the 20 RP vowels: /ɪ i: ʊ u: ɒ ɔ: ə ɜ: æ ɑ: ʌ e eɪ aɪ ɔɪ əʊ aʊ ɪə ɛə ʊə/.

*the 7 vowels of standard Italian: /i e ɛ a ɔ o u/.