The argument is again and again that the value of Bitcoin will increase because supply is limited to 21 million. But this is quite simply wrong. Every time Bitcoin is forked that is in real terms an expansion of the Bitcoin currency beyond 21 million. To some degree the same is true of alternative crypto currencies which will flood the market. There is already more than 1000 cryptocurrencues (
https://en.wikipedia.org/wiki/List_of_cryptocurrencies )
Before investing in bitcoin, i think you should learn about it first. Bitcoin's limited supply is 21million.
I am not sure you yourself understand the problem discussed in this thread here. The discussion is not about bitcoin limited supply (very strictly understood), but rather as you can read in the statement quoted in your own post, "to some degree" about supply of all coins that are derived from bitcoin.