How is the spacing of chairs reinforcement done in a slab?

Chairs usually aren’t used for reinforcement. They are placed to provide your placement distance in horizontal concrete, like slabs.

There isn’t a specific distance, but usually a separation sufficient to prevent large sags. In the US since 18″ on center is so common, a spacing of 3′ (every other crossing) is common. Any spacing can be used as long as the resulting rebar sag is acceptable.