I assume you mean “what are things that one must have if one wants to APPLY to the ivies?”

If that is your question then the answer is the same for any US university in general, with the ivies just having a higher requirement for all 4 of the components below:

1. GPA (school grades)
2. SAT/ACT (standardized testing)
3. Extracurricular activities and academic awards
4. Personal essays and teacher recommendations
5. (optional for most universities) alumni or admission officer interview

Yes with the SAT / ACT do you recommend students to take the SAT and then 2 SATS II ?

And is the ACT compulsory?

You only choose one between SAT and ACT. Either is fine. Some students are better in the SAT; others are better in the ACT. It’s important to choose the right one.

SAT2 subject tests are optional for most universities but if you have the time to do them, then it’s recommended.

How many ACT or SAT subjects tests do uni’s require?

I am thinking to sit the SAT this year and then do the SAT II a few months after, it might be a bit late though.

This is very University dependent. I’d personally recommend doing at least 2-3. If you have 3, then you can not send one for consideration (I didn’t send one of mine because I wasn’t happy with it). Some overachieving students do more than this, but I think there’s probably limited value, as the SAT isn’t really pitched at a very high level - you’re probably better off taking more coursework.