A scatter pays and triggers features regardless of paylines: it does not matter which reel or row it lands in. The required number anywhere on the screen and the feature fires. That is why the scatter is the main key to free spins, and the question of how many you need decides how often you see a bonus.
The plainest version is four scatters in Pragmatic Play’s Gates of Aztec (31 January 2023), with 96.50% RTP and a 5,000x cap. PG Soft’s Lucky Neko (30 November 2020) also needs four and awards ten or more free spins; what the “or more” depends on is left unstated. A bigger grid makes a symbol appear at least once more often, yet gathering four identical marks gets harder, so the frequency cannot be judged without symbol weights.
The scatter that pays by itself
In Yggdrasil Gaming’s Champions of Rome (12 February 2019), scatters carry their own table on top of starting free spins: three pay 20 coins, four pay 300 and five pay 12,000, which is 1, 15 and 600 total bets. The review reckons that this accounts for roughly two points of the 96.40% RTP. In Play’n GO’s Hugo (15 August 2016) there are three kinds. The witch starts free spins and can land on any reel, the chest appears only on reels 1, 3 and 5, and Hugoline shows up only in free spins, but in stacks on every reel.
In Big Time Gaming’s Millionaire Megaways (2022) the scatter count sets the rung of a twelve-step ladder where you start, with prizes from 8 to 50 free spins. Extra spins during the bonus come from scatters on the top horizontal strip: three bring four and four bring eight. Maximum payout: 72,310x.
Apollo Pays, from the same studio, keeps its scatters on the grid for the whole respin chain, so two at the start can become three. PG Soft’s Wild Bandito (3 May 2021), symbols on the third reel other than Wild and Scatter show up in golden frames during free spins.
How many scatters a slot demands, and whether extra spins can be won inside the round, are the first things to check. How scatter pays differ from a cluster game is set out in the guide to cluster pays.