Are you talking about 3.5" drives? You can buy a single 4TB drive and it probably has 3 platters, maybe 4, so I would say that they are at 1TB/platter or better now. That doesn't mean that a 1TB drive is just 1 platter though.
You don't want to squeeze it all on one platter though, because you lose number of heads. A 1tb drive with 2 platters would have faster throughput with 4-heads than the same capacity drive with only 1 platter and 2-heads.