1st attempt - absolute beginner

I have my pressed apple juice from my apples in the freezer and plan is to put into Demi johns together with champagne yeast with bung with an air valve.
How much yeast for a 4.5 litre Demi John? Can you put too much in? What is a hydrometer?

How much yeast for a 4.5 litre Demi John?

That's a pretty strong yeast and will leave you with a fairly dry, wine like cider even with the addition of extra sugar. Good news is extra sugar makes for extra booze.

Can you put too much in?

For that little amount of juice half a packet should do.

What is a hydrometer?

Google it for the in-depth explanation - but it is a tool that allows you to measure the pre-fermentation and post-fermentation sugar density in any brew which helps to calculate the amount of alcohol created by the fermentation process.