Hello Salma,
From my own experience and from many other reports, it seems that the current PMP exam includes far fewer questions than in years past that require one to have memorized and be experienced in using such formulas. From reading between the lines in posts in this forum, I understand that the PM PrepCast course has been updated to reflect this change.
Only someone who is familiar with the entire pool of possible PMP exam questions -- i.e., someone working for PMI and thus sworn to secrecy -- could answer with confidence on just how many formula-based questions might ever feature in a candidate's exam. However, if you are trying to work out how best to allocate your limited study time, learning dozens of formulas by heart is unlikely to be the most useful activity. The devil's advocate answer would be that knowing the formulas makes one a more rounded and versatile project manager, and that's not the same thing as just passing a certification exam.
When I was preparing, I did acquire the Formula Guide and also learned and practiced using many formulas that I had encountered for the first time in PM PrepCast materials. While very few of these featured in the PMP exam, I still don't consider the time I'd spent getting to know them as wasted.
Wishing you all the best in your continuing studies!