How many reps you have to do with lighter weights to feel "ready" probably has to do with technical proficiency, personal preference, and the lift being performed. I tend to stick to a relatively minimalistic approach.
However, squatting requires a little bit of preparation. I sit in a squat while holding a lightly loaded barbell on my knees for a few minutes. This allows me to stretch my knees, hips, ankles, etc. I then perform 10-15 air squats, gradually increasing the speed of my descent. I then perform 10-15 reps with the bar.
If I were working up to a heavy single, I'd then do something like..
If I wanted to do triples in the low 400's, I would do everything the same up to 375, then maybe do another single at 395, and then start working at 415 or something.
The deadlift is very simple for me and requires no prep work. I simply load the bar with a plate and go. E.g..