There is spellcasting with specific spell reactions, like counterspell. The game doesn't have the warcaster feat, due to their SRD limitation - so not opportunity attack casting, but you can ready action a spell; it limits you to cnatrips for that, so as to not waste resources unexpectedly or uncertainly - that is one limitation of the system there (you can't, for example, 'Ready' "Fireball, when the fourth skeleton makes it around the corner").
In terms of multiple counterspellers in the same moment, I'm not 100% certain, but the way I believe it works is that the first person to input will be the one that is ultimately used up, if multiple characters say yes, but definitely only one player's spell and reaction is consumed, however it determines it - that much I can confirm.
