Marketcap:- this is the total worth of bitcoin. Like in US dollar, the total funds of people that bought bitcoin in USD which is currently around $580 billion. The more people that buy bitcoin, the marketcap will increase. The more people sell Bitcoin, the marketcap will be decreasing. The marketcap is proportionate to bitcoin price.
Demand:- as explained above, the high there demand, the higher the marketcap, the higher the price.
Supply:- the higher the supply, this will affect he price, but the supply of bitcoin is limited/finite and not much but only 21 million bitcoin in total while over 18 million in circulation now.
BTC price keeps getting higher but that's just because the number or circulating bitcoins decreasing
I don't know what that mean?
No. Decreasing supply is one of the reasons butarketcap is the main reason now.
if I buy 1 BTC now what guarantee do I have that that circulating supply goes down I wont have less than 1 BTC?
The supply of bitcoin in total can not decrease.