Solid line = floor (spellings that unambiguously belong to the group). Shaded band up to the dashed edge = ceiling (the most it could reach if every ambiguous spelling counted here).
Sincere has been climbing in recent years. All-time it's been given to 15,375 U.S. babies, the 817th most common name by sound today, peaking at 825 in 2022, with 681 in 2025.
By sound, Sincere also includes the spellings Syncere.
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).