For each theme, how many quotes/events do you recommend me to prepare?
I often make my notes over detailed and way too in depth, how do you decide between which events to cut and which events to keep? 
I struggled with this too in high school! I would find so many good quotes in my text and I wanted to use them all haha.
You've just got to look over your stuff and be pretty ruthless - think, "can I use this example in lots of different paragraphs/essays? can I adapt it?". If it's really niche, maybe consider using other ones, unless you're confident you will either have an opportunity to use the niche example

And also, try to keep your quotes pretty short and sweet, easy to place into a sentence, etc.
Depends how many themes you have for your text, but I probably had about 4-5 key themes I think? And maybe 4-5 quotes per theme, plus some general quotes I knew I could sneak in. Honestly, you'll be suprised how many you can remember! I managed to put quotes I didn't deliberately revise into my text response in the exam last year!
Maybe you can cut things down by planning out heaps of essays (just break down the prompt, figure out what you'd write your 3 or however many body paras on, and dot point the evidence you'd use) - a) you're revising for the exam this way, b) you're putting away heaps of quotes and examples into your brain without even realising, c) you're finding out which examples/events really are able to be used, and d) you're gonna make planning and writing your essay in the exam a quicker process because you'll be able to just combine aspects of paras from different prompts and change them around a bit - patterns repeat! (if that makes sense)
Might be worth having a chat to your teacher to ask them as well? They'll have guided a fair few students through the English exam in the past, so will no doubt have some really good wisdom to share

All the best!