The Indian Super League started in 2014 with eight teams and expanded to a 10-team league with the addition of Bengaluru FC and Jamshedpur FC ahead of the 2017-18 season.
The now-defunct FC Pune City were replaced by newly-formed Hyderabad FC and Delhi Dynamos rebranded themselves as Odisha FC, keeping the number of participants same in the 2019-20 season.
I-League champions Mohun Bagan and ATK merged to form a new entity ATK Mohun Bagan but that still did not increase the number of teams in the league. However, for the first time ever, ISL will have 11 teams taking part when they confirm the entry of East Bengal.
Meanwhile, I-League is also set to have 11 teams despite the exit of East Bengal.
Mamata Banerjee, the Chief Minister of West Bengal, announced on Wednesday that the Red and Golds will take part in the upcoming season of ISL. And it looks like India's top-tier football league will have an odd number of participants this season - a scenario that is uncommon but not completely unheard of.
While most leagues consist of an even number of teams, this anomaly is likely in situations where there are new additions. Two years ago, The Major League Soccer (MLS) had 11 teams in the Eastern Conference and 12 clubs battling it out in the Western Conference, making up a total of 23 clubs for the regular season. For a three-year period from 2012, that number was 19.
The Turkish Super League, which generally consists of 18 teams, has decided to cancel the process of relegation due to the Coronavirus pandemic and thus, will allow 21 teams to participate next season. This was because the FA took into consideration the impact of COVID-19 on clubs who were supposed to be relegated. As it stands, the 21-team arrangement is just for one season.
There are also two examples from Africa, both of which occurred due to very different reasons. Cameroon's top division Elite One was split into two groups of nine teams. Kenya's 2019-20 season was reduced to 17 teams from 18 after just three matches because former champions Sony Sugar was expelled from the league for missing the first three games.
Mexico's top division Liga MX is split into two campaigns every season - the Apertura and Clausura. A detailed explanation of how that works can be found here. While both consist of 18 teams each usually, the 2019 edition of the Apertura had 19 teams and lasted 19 weeks.
The J3 League - Japan's third division - had 17 clubs in the 2018 season.
A league with an odd number of participants using a round-robin format would ensure that at least one team get a week-off every round.
The 2020-21 season of ISL is set to be played behind closed doors and the teams are likely to be grouped to be played in select venues. With East Bengal's potential entry, it will be interesting to see how the league calendar turns out.