Actually I have this similiar problem too with Arcana Magi. It's a good idea to bring them in one at a time, and here's why, I your early you're just establishing the direction your taking your main character, chances are you're going to have your main character or meet somebody that wasn't in your original plans. That may affect the way you bring in the other characters. You may also find yourself introducing a character and you don't like the role or direction your taking that new character. So you are able to make adjustments to the new character without dmaagin what you had written so far.
If you introduce all your characters in one or two chapters, you have to stick with who they are and how they act. You will find yourself five chapters deep saying, hey this character could have done this and this character could have done that, but it's too late. Your readers now know who your characters are and changing mid story will turn them off.
I'll give you an example of what I went through in Arcana Magi. I have four characters who are supposed to be Sentinels, I never intended in the beginning to have the Character Reya Everheart, but there she was unexpectedly in the story. I worked her into the story nicely and thought about my next character to introduce. When I wrote the character in, that character was out of place and slowed the story down, not only that but I needed to start introducing the other Sentinels. So a nice rest and meditation later and I thought of how to make Reya relevant, add the next Sentinel, and introduce the character I originally ommited. But in the process I also shuffled the roles of the three remaining Sentinels I have yet to introduce and what I want to write in my story looks a lot more fluid than what I was originally going to write. Of course it also means going one more chapter without introducing the next Sentinel but that chapter will serve to set up the appearance and maintain the stories pacing.
It's not easy having an ensemble cast in literary writing, but a good balance in the storytelling and keeping the Point of View on the main character, should keep your story from flatlining.
Hope this helps.