Hello Centcel,
I am not sure if I understand your question about the Demo accounts. However, if you want to have a couple of MT Demo accounts on your computer, you need to download MetaTrader from different brokers and install those on your computer. But this does not mean you have to trade on different brokers. You can log in to another broker’s account. For example, if you have downloaded the platform from broker X, you can log into this platform with the account of broker Z. So you can have 2-3 MT installed or even more, but use the same broker.
A common mistake is that people open a few accounts on one MetaTrader and they switch between them. That is fine for manual trading, but when trading with Expert Advisors that will disable the EAs from the accounts that are not active.
Cheers,