Most of them, to what reffer?

Can you please tell me to what reffer "most of them" here?

price of a single unit of a currency was rising. profitability of mining was also increasing. it resulted in investments in next machines necessary for excavating a greater quantity of a particular cryptocurrency. with the passing of time the extraction of cryptocurrency began to be harder. as we know - most of them have a determined number of units that can be in circulation. we call this the limitation of supply. it results in limiting the quantity of a particular cryptocurrency on the market.