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).
Isabel is down from its peak but still widely used. All-time it's been given to 269,257 U.S. babies, the 179th most common name by sound today, peaking at 8,971 in 2007, with 3,853 in 2025.
By sound, Isabel also includes the spellings Isabelle, Isabell, Izabelle, Izabel, Isobel.
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).