The Hermitage tells us that in the decks before Waite-Smith, the Fool is almost always unnumbered. There are a few exceptions: some old decks (including the 15th-century Sola Busca and the Rider Waite) label the card with a “0”, and the Belgian Tarot designs label the Fool as “XXII”. The Fool is almost always completely apart from the sequence of trumps in the historic decks. Still, there is historic precedent for regarding it as the lowest trump and as the highest trump.
Traditionally, the Major Arcana in Tarot cards are numbered with Roman numerals. The Fool is numbered with the zero, one of the Arabic numerals.