The "Maximum Number of Devices" is the number of separate devices permitted to register on this website under your name. Only the computers you physically log into, and use to register, will have update access to your data on the website. No other computers can have access.
If unsure, leave the Maximum Number of Devices at the default = 2.
Do not put Maximum Number of Devices at 1 unless you enjoy high risk. With a maximum of 1 computer permitted to register (the computer you are now using), nothing else can access the website. So if you lose the computer or the computer breaks, you will never ever be able to access the website again and you will LOSE ALL THE DATA YOU ENTERED!
The best and most secure method is to put Maximum Number of Devices at 2 or more and ensure that exactly 2 or more computers always registered. That way, if one computer breaks, you can always use another computer to access the website. You can then de-register the broken computer and add a new computer thereby still having 2 registered computers.
Note that if you put the Maximum Number of Devices at 2 and only register 1, that works. If you lose the device, you can always register another computer because you have the extra slot. However, this method leaves a huge security hole! Anyone that knows your name, email and phone number can register their own computer under your name and then have full access and privilege on the website as if they were you. Again, if you lose your 1 device, you have the same problem as just mentioned - YOU LOSE ALL YOUR DATA.
This explains why for security, the Maximun Number of Devices should match the number of devices you plan to register. Do not leave open device slots available for potential hackers.
Changes to the Maximum Number of Devices, as with all data, can be done with any registered device.