Re: How many new characters should I introduce at a time?
I had much the same problem, so I started off my first manuscript by writting 'diary entries' by each of my supporting characters. It's easy to introduce the protagonist as they become the center of your new world, but the supporting characters become lesser versions of the main character rather than individuals.
The diary entries allowed me to iron out the character traits and speech patterns as well as their driving force for helping (or hindering) the protagonist.
By the time I started working on my third story, it became easier to keep all the characters seperate in my mind...well, that and there's really only 3 characters who need to have deeper motives than an extra on a movie set. lol.
It's never too late to do diaries, and I found them to be incredibly helpful on re-reads as well.