According to the Hurlingham Rules (2015), "A match may be played over 4, 5 or 6 chukkas as stated in the Tournament Conditions." In other words, it varies from tournament to tournament – but there are never more than six chukkas.
However, the International Rules (1982, updated 2010) state that "The standard duration of a game shall be 6 chukkers. The number of chukkers in a game may be varied by the tournament committee. There may not be more than 8 chukkers, not including extra time."
As far as I can make out, the Hurlingham Rules apply in the UK, Ireland and the United Arab Emirates – and possibly elsewhere. The International Rules are drawn up through a co–operative agreement between the Hurlingham association and its equivalents in Argentina and the USA.
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