Like saying how is gold supply finite if you can break a piece of gold in half. When Bitcoin forks the value is split between the two forks. If the market cap of Bitcoin before the fork is $10 Billion, the combined market cap of both sides of the fork afterwards will still be $10 Billion. There are double the number of coins because of the fork but now each person who had one coin has two and each coin is worth a fraction of what is was before. It's an increase in supply but an increase that is perfectly distributed among the previous holders.