But then i sell my core bitcoin for Segwit bitcoin and the amount of coins stays the same right? How can i increase my coin then?
you are mixing things up. the "battle" is between segwit (core) and bucoin (bu). bucoin will be nothing more than an altcoin. if you calculate from there, then it's basically free money in case the fork creates 2 seperate blockchains and you sell bucoin.