Using Console command
set playerfollowercount to 0 my old follower leaves. Only Serana doesn't leave and I need one more than Lydia. I know there are hundreds of mods which can fix this but My friend don't want to use them
Well, If this is the case you could always try using a mod that would increase the amount of followers, then you wouldn't have an issue try this. This is usually a glitch caused by starting a quest where a npc waits for you. I myself have had this glitch and I just used an older save to before this had happened.