A totally phonetic English spelling system would, by some accounts I've read, require anywhere from 89 to over 120 different characters.
That's to accommodate everybody but the guys in Jamaica, eh!
I think the phonetic spelling systems that have been developed to help foreigners learn to pronounce English correctly get by with a lot fewer than 89 symbols. The main problem is that there are too many vowel sounds--the letter A doesn't just do service for "long A" and "short A" but for several different "short A" sounds, and the vowels in the written language are mostly diphthongs in reality.
I think the number of symbols needed is somewhere between 30 and 40--fewer if you use two-letter combinations like "sh" for the sound usually spelled "sh" in English.
Systems with 89 or more symbols are usually syllabaries (like the Cherokee syllabary invented by Sequoyah, or the Linear B syllabary used by the Mycenaean Greeks).