The SmartSnugg monitoring system is made up of a SmartPebble, SmartBridge and the SmartSnugg App (which is free to download and use on iOS and Android). The SmartPebble which is positioned inside the SmartSleeper is waterproof, machine washable & has a 9-month battery life. Batteries are replaceable and can be changed when needed.
You do not need extra Pebbles or an extra Bridge. One Pebble and one Bridge is all that you will need to monitor your child. The Pebble can be easily moved between Sleepers.
However, you might find it convenient to have more than one Pebble, as you can then leave the Pebble in the Sleeper for several months as you alternate Sleepers. This is an option because the Pebble is machine washable and uses so little energy that it has a 9 month battery life!
The main reason that you'd want an extra Bridge would be if your child regularly slept somewhere other than their main bedroom. In that case you could set the Bridge up at the second sleep location each time or you could have a second Bridge permanently set up there and ready to go.
You do not need buy Blankets, they are an optional extra. However, they are great value for money and will seriously increase the value you get out of each SmartSleeper.
For example, if you buy 2 Sleepers (different TOGS) and 1 Blanket (any TOG) you no longer have 2 Sleepers you have 4! 2 Sleepers and a Blanket will cost you less than 3 sleeping bags of an equivalent quality but will give you far more options!
And if you had 2 Sleepers and 2 Blankets you don't have 2 Sleepers you have 6!