It seems to look like a fingering number but I don’t have 7 fingers and the note is already marked with a fingering number. This is from Chopin's Fantaisie-Impromptu.
This is a tuplet, indicating that seven notes in the right hand are to be played in the same time as the regular six in the left. You have probably already encountered triplets, the most common tuplet, earlier in the same piece.
Here it is in another edition which makes its separation from the fingering clearer:
2@replete Good answer. How can you not be fond of this piece? Just kidding, I know it's individual preference, but I just personally love this piece.– EffMar 27, 2019 at 7:48
2How hard is it to play? It sounds crazy to have the right hand play 16.666667% faster than the left hand, but I have no clue about piano. Mar 27, 2019 at 13:00
3Toads of the Short Forest, " Each musician follows a different time signature, as Frank Zappa explains to the audience. " I'm trying to find the exact text, but there were at least 5 different ones, and Zappa ends with "... and the saxophonist blowing his nose" . I would have just as much trouble trying to play 6 against 7 as in this Chopin piece. Found it! "At this very moment, on stage, we have drummer A playing in 7/8, drummer B playing in 3/4, the bass playing in 3/4, the organ playing in 5/8, the tambourine playing in 3/4, and the alto sax blowing his nose." Mar 27, 2019 at 13:05
1@CarlWitthoft - I've depped in bands like that. Except that the numbers were often supposed to be in 4/4...– TimMar 27, 2019 at 19:36
1@EricDuminil If you were to play it with that kind of robotic mathematical precision, yes, it would be pretty tricky. But this is Chopin - playing it that precisely would kill the soul of the piece. The performer is expected to interpret the rhythm in a more organic way, without worrying about the exact ratios of note-length between the two parts. Mar 27, 2019 at 20:24