It rather depends on the Monarch.
English and British Kings were unable to Levy taxes on their own behalf. The last one that tried got his head cut off!
In order to raise taxes (Usually for a foreign war ) they had to call get the agreement of Parliament.
However there were lots of other ways to raise money, monarchs had the power of patronage, granting rights, land, monopolies, titles and there were lots of opportuntities for kick-backs. All in all what we would consider corrupt practices.
There was also good opportunities for share of booty in foreign wars.
And earlier monarchs made money by founding bouroughs and charging rents and taxes on them. King John founded Liverpool in this way. They could then raise money and buy the independance of the bourough back from the King.
Of course the most notorious royal money making adventure would be Henry VIII's dissolution of the Monastries. It's difficult to overstate exactly how rich some of these institutions were they owned nearly 1/6 of England with all the farms and enterprises on them