I have no idea of the percentage of English speakers, but I didn't have a problem. This Quora answer [1] mentions that younger people are more likely to speak English, and I saw that but the interactions with middle-aged people was OK too. There's usually a way to get by. With the professional class there, their conversational English was excellent.

[1] https://www.quora.com/How-many-people-speak-English-in-Iran