Grrrr. I won't ask what YOUR architect says, because you probably don't have one. WHY do people skip this VITAL step when they make the biggest single investment in their lives, and then constantly find themselves in situations like this?
Now that my rant is over...you are entitled to what is shown on the plans for the house and what is written into the specifications, no more, no less. If those documents are vague, or favor the builder, now you know what architects are for.
Without knowing anything about the house, it's impossible to comment. Is your house EXACTLY the same as the model you saw? Does your contract read that your house will be like the model house in every detail?
The kind of beams used depend on the spans and the loads they must carry. There is no way, based on no information, that any comment is possible. Perhaps for the span you would need and the loads on the beam, a single beam would have to be too deep to allow proper headroom under it, but there is no way to know.
Sorry I can't be of more help.
Architect (NY) and Home Designer (PA)