Here is a fun little word puzzle. The task, for as many letters of the alphabet as possible, is to find words that have the sound of the name of a letter at the start but which don’t start with that letter. To give one example, “euphemism” starts with the sound of the name of the letter U but starts with an E.
I was able to establish
4, 5 numbers and 17, 19 20 letters to meet your criteria. 
M: important, embark
N: inn, entrance
1: Special thanks to @Stiv and @Simon Jenkins for pointing out an additional number and three more letters!
A: Can élan and éclat be counted? As per Oxford and MW dictionary, they are pronounced as ā-ˈlän (ay-lan) and ā-ˈklä (ay-klah) respectively. However, some might argue that they are French words. C: seam, seizure, seem, sick, F: any word starting with "ef-" e.g. effluent, G: jeans, I: any word starting with "eye". Also, eyrie. Can be pronounced as ˈaɪəri or I-ri. K: cable, L: any word starting with "el" e.g. elephant, elevate, M: any word starting with "em" e.g. embarass, N: any word starting with "en" e.g. entropy Q: any word starting with "cu" e.g. cube, cupid, R: any word starting with "ar". e.g. argon S:any word starting with "es" e.g. establish, U: the pronunciation of the letter forms a word - ""you"; Eucalyptus, Euphoria,
Europe(excluding because in British English, it is pronounced as "your-ope". Thanks @alephzero). X: any word starting with "ex". e.g. exist. Y: Wide
Does Pseudonym count?