Yes, first target is 4, then 10 (usually that is enough for retirement, and you will need to hire a property manager), and then if you really like it....and employ couple of other people, 50!
These are not my idea exclusively, sort of a standard template.
It's great, I was of the mind that paying the primary mortgage off was best too but the idea of getting a 2nd 3rd etc then using them to pay of the primary, Its awesome!
I need to learn more standard templates. :OD
https://www.biggerpockets.com/podcast
https://www.biggerpockets.com/podcast_episodes?page=32&series=BiggerPockets+Podcast