This is documented after all. There is a 500 character limit and a max length per keyword of 30 characters. I have a few forced keywords at the start, which lowers it to 425.
I’ll ensure to enforce this when entering data. Also, keywords with spaces are enclosed in quotes which counts against the overall total.
12 34,56 78 becomes “12 34”,”56 78” with a total of 15 characters.