Twelve base characters can form Unicode keycap emoji sequences: the digits 0 through 9, plus the number sign (#) and asterisk (*).
A keycap emoji is not usually a single dedicated code point. Its sequence combines one of those base characters with an optional emoji presentation selector, U+FE0F, and U+20E3 COMBINING ENCLOSING KEYCAP. For example, 1️⃣ is built from the digit 1, the variation selector, and the enclosing-keycap mark.
The count is therefore 10 digits + 2 symbols = 12. The base characters themselves are ordinary Basic Latin characters, which is why the same digit or symbol can appear as normal text in one context and as a colorful keycap in another. U+FE0E can request text presentation instead of emoji presentation where supported.
A common mistake is to count only the ten digits or to confuse keycap bases with regional-indicator letters, which are used in pairs for many flag sequences. Keycaps are sequences, not twelve newly assigned standalone emoji characters.