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."
So there may be eight chukkers (or chukkas, if you prefer), but this is not standard – even under International rules.
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.
At the end of each chukka (except the last one in a game), play continues for 30 seconds or until there's a stoppage, whichever is sooner. So the length of a chukka is sometimes given as 7 minutes 30 seconds.
© Haydn Thompson 2022