A character count is a starting point, not a visual fit test. Count the proposed label with Character Counter, then place the longest realistic version in the actual button, form, print layout, or filename field.
Spaces, punctuation, emoji, and translated text can change the result. Check at narrow width and large text settings. If it clips, keep the meaning and shorten supporting words rather than replacing a clear label with a cryptic abbreviation.
Where will this label appear?