Hello @fang3412 welcome to the Community.
So originally (to not bite off more then they could chew) WarHorse Decided the best approach to successfully fund and make their game would be to take the story they intended to tell and break it up into three acts, each act being a whole game in itself.
Your forum badge is not showing so I do not know what tier you pledged so I will go with he assumption of Baron.
With the Baron pledge you will get Act 1, A full game with a current estimated play time of 50+ Hours.(Along with other things that are listed in the pledge description)
How much of the game will be accessible in beta has yet to be disclosed. With the 0.5 alpha update coming soon here in October, WH usually makes an accompanying video update that will give us brief plan of whats to come later.