1 ps4 has ps plus ,how many consoles can be online

[sav1]

If one playstation 4 has playstation plus on it, how many other playstations can still get online without having to buy another ps plus .

As it stands, Main playstation 4 has ps plus, other ps4 doesn't have ps plus but it can go online and play bought games only.Can I buy another ps4 for my daughter for a xmas present and she will be able to play online aswell or do I needs to buy another ps plus membership



Thanks