I hope it's free like in granblue fantasy. if there'll be any limitation, it'll probably be some kind of energy or action point (AP) costs needed for each map which normally will increase the longer you go in the story (can't imagine any mobile game without AP system nowadays)
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also I literally just made a new account here because I've been having my fair share of joy and salt in mobile game since I played granblue fantasy back from last year, and having similar situation in fire emblem franchise game will be a hillarious experience for me personally
btw, I don't know if this is real or not but I find the english twitter for FE heroes
https://twitter.com/FEHeroesApp