PokeAlert produce very good result with only 5 accounts on a radius of 500m. But then again it will depends of your need. The more big the radius the more account you will need so that scan complete in a decent amount of time.
The majority of PokeAlert user uses between 5 and 10 accounts and thisis plenty enough for a fun hunting tim around you.
Again because of security measures you cannot start and stop scan in different locations. If you start a scan to far from where you stopped it your accounts will be "speedlimited" and will NOT show any Pokemon for a limited amount of time.