Everything is in the title.
For example, we know that the letter A has 9 sounds in RP;
The short “a” as in at. /æ/
The long “a” as in ate. /eɪ/
It can be "silent" as in boat. /əʊ/
The schwa as in tuna. /ə/
The short “e” as in many. /e/
The long “o” as in water. /ɔ:/
The short “i” as in garbage. /ɪ/
/ɑː/ as in art
The short "o" as in swan. /ɒ/
Where can we find the percentage of occurence of the each nine cases ?
PS : The "short" and "long" descriptors are common in education in English-speaking countries : https://en.wikipedia.org/wiki/Vowel_length#%22Long%22_and_%22short%22_vowels_in_orthography