Extras are just basically like the ensemble of the series, they’re the lines in the background, or the characters that only appear for a second. Multiple people can be casted as this role, probably up to five, but no more. Extras will not always be needed in every single episode, but if they are, whoever lands the role will be notified, there is also always a chance to be promoted to a higher role later on, if another role is needed in the series.