Satoshi was a clever person and he anticipated every aspect of Bitcoin. The number of btc will be 21 million only but do keep in mind that 1 btc has 100 million satoshi so total satoshi will be 21 million × 100 million that's a huge number 210 trillion satoshi  to be exact. Imagine when 1 satoshi is equivalent to 1cent the market cap will be 2.1 trillion dollar and if 1 satoshi goes to 1$ each 210 trillion market cap. But in my opinion 1 cent per satoshi is achievable in near future ( may be within a decade). So 21 million btc are more than enough and i think there will be no need for a hard fork to increase the supply limit