Are you writing a series of books with the same set of characters, or do you intend to write more books in the same genre after killing off previously used characters?.
My intention has always been to write a series of books using existing characters.
Agents like to hear that authors intend to write more than one novel. That may influenece their decision on whether to represent a budding author. The alternative is self-publishing.
My tutor said she knew someone who had two projects on the go at the same time. She had two desks so she physically got up from one book and moved on to the other. I admit to starting a bit of my next book before my series is complete, but it's more notes than actually writing.
I certainly didn't imagine my first story idea would be a five book series. I am now of the opinion no one should set out to write one of those. If your characters are strong enough and there is enough plot to go after the first book, then so be it, but I don't think you can really know if your MCs will have the legs until you've travelled at least part of the journey with them.
I never intended to write more than one (historical action/adventure) but having got my two MCs through one battle thought 'why not?'
The second story took ages but is now almost complete, third and fourth have detailed outlines and I have ideas, in various stages of work, for another six. At least.
It takes so long to write decent characters (I hope mine are that) it seemed a shame to just dump them right at the beginning of a seven-year conflict. I know one of my favourites will die in Book 4 - I agonised for ages over whether I could tell as good a story without killing him off - but it has to be :(
How many'll get to all the way to Waterloo is anyone's guess.