According to Katharine Sands, literary agent and author of Making the Perfect Pitch, "series" is not a magic word with agents. So in your initial query letter, focus on the first book. Within the first paragraph, agents should get a vivid sense of your voice, your character, your setting, and your story's conflict. You have a far better chance of conveying all of this if your letter is focused on one plot. Furthermore, an overly ambitious letter may come across as naive and unprofessional. If your letter does its job, agents will know that they are dealing with a real writer, and they assume that real writers have more than one book in them.
When you sit down with an agent, they will want to know then if you have a series planned. At that time, those other books will very much be to your advantage.
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