Although it's entirely possible that there is some deeper significance in the number of Berúthiel's cats, it should be noted that she had
ten cats, not nine; and that ten is a number that tends to spring to mind quite naturally without any deeper meaning being intended. As for there being nine black cats, Tolkien probably wrote Berúthiel's story at about the same time as the appendices to
The Lord of the Rings, when the idea of nine black servants was strong in his mind. The only reference to numerals in his letters concerns the convenience of duodecimals (multiples of twelve) and some Elven numerals for them that he had invented.* I think that a numerologist would have had more to say about numbers than that. More tellingly, Shippey's literary biography
J.R.R. Tolkien: Author of the Century makes no reference to numerology. Perhaps there are some other hints elsewhere.
I noted Tolkien's apparent dislike of cats some time ago, and started
a thread on the subject. Consider that a shameless plug.
*Letter #344 (23 November 1972)