been Charged with a section 20 assualt will i go to prison?

[Decstarmm]

this is my 1st offence it happened a year ago i was cautioned for striking a lad after his friend hit my friend inside a kebab house him and his 11 friends came in the kebab house there was a silly arguement his friend then punched my friend there was 3 of us total and 12 of them they then waited for us outside the kebab house i was angry so i went out side and hit 1 of them i then ran down the rd and another one chased me down he held my arm and said i was not going anywhere at which stage i then hit him once in the face knocking him out then the police came and i was arrested given a caution and let go i have now been brought bk in for the offence as it turns out there was a mix up with the names coz they were brothers so now i am being charged with section 20 bcoz when he hit the floor it caused a blood clot to the front of his brain i feel horrible and i am terrified i will go to prison for this offence im going to crown court what am i looking at i would really love some advice thanks