By design, a total of 21 million Bitcoins will be created over 100 years according to a logarithmic release function. At the time of writing, just over 13 million bitcoins are in circulation, meaning that an additional 8 million bitcoins will be mined over the next 95 years 
. Given this timescale and the decelerating rate of increase of the coinbase, the supply of bitcoin can, for practical purposes, be assumed to be constant. 
But you didn't explained why Bitcoin is a volatile currency, you might have explained why Bitcoin is expensive now as there is only 13 million in circulation. Having a limited supply does not explain the volatility but the buying and selling pressure can, every transaction of buying and selling will be reflected to its price chart in which we can determine the price trend. . Given this timescale and the decelerating rate of increase of the coinbase, the supply of bitcoin can, for practical purposes, be assumed to be constant.