There are currently 17 × 65536 code points (U+0000..U+10FFFF) in Unicode. UTF-32 could theoretically encode up to a hypothetical U+FFFFFFFF and still be fixed-width.
Note that, at present, only 4 of the 17 planes have defined characters (Planes 0, 1, 2, and 14), two are reserved for private use (15 and 16), and an additional is unused but is thought to be needed (Plane 3, the TIP for historic Chinese script predecessors). Four planes appear to be sufficient to support every script ever written on Earth, as it's doubtful there are unidentified scripts with an ideographic repertoire as massive as the Unified CJK ideographs database.
We are very unlikely to ever fill up the current space of Unicode, let alone the plausible maximum space permissible by UTF-8, let alone the plausible maximum space permissible by UTF-32.
Note that, at present, only 4 of the 17 planes have defined characters (Planes 0, 1, 2, and 14), two are reserved for private use (15 and 16), and an additional is unused but is thought to be needed (Plane 3, the TIP for historic Chinese script predecessors). Four planes appear to be sufficient to support every script ever written on Earth, as it's doubtful there are unidentified scripts with an ideographic repertoire as massive as the Unified CJK ideographs database.
We are very unlikely to ever fill up the current space of Unicode, let alone the plausible maximum space permissible by UTF-8, let alone the plausible maximum space permissible by UTF-32.