Where a plus range starts varies by brand, which is part of why sizes feel so inconsistent at the top of the chart. Some brands begin at 14, others at 16 or 18, and a few use 1X/2X/3X alpha sizing alongside numbers.
Good plus-size grading re-shapes the pattern rather than blowing it up uniformly, since body proportions change as sizes increase — a detail that separates well-fitting plus ranges from poorly graded ones.