This is a simple plugin for the Flipper Zero. It gives you access to a counter which you can increment and decrement using the up and down buttons respectively. Long-pressing the OK button will enable vibro feedback for adding, and long-pressing the back button will clear the counter.
The easiest way is to download the counter.fap file from dist, and place it inside of the apps folder on the SD card of your Flipper Zero device.
For developoment, I'd recommend using flipperzero-ufbt, which is a lightweight tool for quickly building and testing Flipper Zero applications.