The ramp-up period controls how long it takes to ramp-up to the full number of virtual users selected. If 10 virtual users are selected and the ramp-up is 100, it will take 100 seconds to get all 10 virtual users up and running. Each virtual user will start 10 seconds after the previous virtual user.
Ramp-up time/virtual users = seconds per 1 virtual user
Based on the examples you mentioned:
500/100 = 5 seconds
1000/100 = 10 seconds
* Note: The ramp-up period is automatically adjusted based on the number of virtual users you select, but it can be manually modified.