If you want to build a mobile application to detect and count faces in a picture, you will need to learn at least a bit of Artificial Intelligence and then code to do all the requirement. This will be quite a bit of work to do.
But, there is another way - an easy way.
Whether it is Android, iOS or even desktop, you do not need to learn any AI. Using one of the Sulata.Net's APIs, you can do it in four simple steps.
For instance, you need to detect faces in the following picture.
You just need to send an API request to a URL like this.
You will get a JSON response like this.
The above JSON response says that there are three faces in the picture and they have been marked with rectangles in output URL.
When you call the output URL, you see the following picture.
With this little line of code, you can make quite a lot of applications, like a People Counter.
There are many other, ready-to-use APIs available at www.sulata.net. You can just grab the idea and launch your own app or use it for any of your requirements.