It depends on how you plan to power them. If you want to use the onboard regulator with a 9V supply then you'll use a lot more power than connecting a 5V supply directly to the 5V pins.

From a quick search it seems a Uno running on a 9V supply takes 50mA. Multiply this by the number of devices and you'll get the power requirements. You say > 50 but without the exact number it's impossible to calculate. At 50 it would be 50mA x 50 = 2.5A.

If you connect directly to the 5V pins then it removes the (inefficient) voltage regulator.

Also there are various versions of Uno such as the Nano, pro mini etc. so you need to decide which version you are going to use.

With a 9V battery you will power a standard unmodified Uno for less than a day.

Also remember if your code is doing a lot of work then the processor will use more power.

Personally for this number of units I would use a 5V regulated PSU (you can buy them cheaply on AliExpress for example) then solder flying leads to a piece of Veroboard to the 5V pins on each device.