Ummmm depends? I think for a person doing this causally, 2 projects should be the limit. Saying that, I'm not following my own advice, cause I'm working on 2~4.
I currently have two, but they have different upload rates, so I'm I have time to work on one before touching the other. I pause one of them after each arc to further plan future ones while the second story has shorter chapters and requires less brain power for me to write. I keep notes of both and organise the chapters so I can always remember what I have/will write.
But yeah, the basics advice I have is.
- Keep notes (I do them online cause it's easier for me to keep track of them) (They don't need to be extensive, just what ever works for you).
- Keep upload rates different (If you have a schedule), dealing with two deadlines that are happening at the same time is much harder than just one.
- Some people can do 1 chapter of one then switch to one chapter of the other, others do them in batches, others just write whatever they are motivated to write at the moment. I can't really say what will work for you, cause everyone's different.
- Don't keeping making new stories each time you reach a block for all your current stories. This becomes easy to do once you start the first step and make a second story. But this is a terrible idea (Making a short stories is fine, but doing longer stories is a big no no.)
Yeah, hope this helps.