Your fingers need to be numbered primarily to help identify which fingers should or could be used on a certain note, or set of notes.
For example, typically when reading from sheet music the composer will indicate some of the notes with the use of numbers instructing you of which finger should (or could) be used. There are only usually a few notes throughout the piece that are given a suggested finger to use. The reason for this is that it’s usually straight forward working out which fingers are best to use, so the composer would typically only indicate some of the notes which are not so obvious. It’s also important to remember that these finger numbers are not mandatory, but are more for guidance.
However, more often than not these numbers are correct, and you need to use them. Others may disagree, but I very rarely ignore the suggested fingering because you’ve only got ten of them, so it’s very likely that they’ve indicated the best one for you to use. Now and again though you may find it better to use a different finger, but it’s important you check this with your tutor who would help you with this during your piano lesson. If you teach yourself, then I would suggest sticking to what they have indicated otherwise you may fall into some bad habits that you can’t get out of.
Scales are extremely important when it comes to gaining a great knowledge of which fingers to use. By practising scales alongside pieces, you will find that using the right fingers comes naturally as you progress.
Your fingers are numbered from 1 to 5 on each hand. So rather than from 1 to 10 which would make it harder, it’s split between each hand. The thumb is number 1; the forefinger is number 2, the middle finger number 3, and so on.
From a piano teacher’s perspective it’s also really helpful for a piano lesson, as you can instruct your pupil as to what finger to use. It takes a little getting used to, but once you know each finger number it makes learning a lot easier.
Here is a diagram to show you how it works: