if the there is a huge number of supply the demand is decreasing,
demand doesn't drop because of supply. demand is demand. think of demand as the number of people willing to buy the coin, this number depends on a lot of factors and change based on them. for example the main reason is usually speculation reason, simply expecting price to rise or fall due to news, pumps,... then there is also usability for example using bitcoin as store of value, use it to buy stuff,...
the way supply relates to this and then determines the price is that when there is more coins than there is buyers the price continues going down. imagine there are 100 people wanting to buy a coin that has 10 coin supply versus a coin that has 10,000,000,000 coins.
and supply in this case is not just the circulating supply of a coin, it is the coins that hit the market. for example a lot of coins (like XRP and ETH) have "unaccounted for supply" which is a lot bigger than their circulating supply. for example ripple has 100 billion total with 39 billion released but a small amount actually in the market. ETh has 97.4 million coins while having 70 million premined and lots of it locked/burned in smart contracts.