Time Stop gives the caster a variable number of "free" turns (marked in bold):
You briefly stop the flow of time for everyone but yourself. No time passes for other creatures, while you take 1d4 + 1 turns in a row, during which you can use actions and move as normal.
When I cast Time Stop, do I know exactly how many turns I'm getting, thus allowing me to plan exactly the sequence of actions I will take while time is stopped? Or do I only know that the flow of time will resume some time between the end of my 2nd turn and the end of my 5th turn after casting it, requiring me to plan for this uncertainty?