In principle, there is no direct relation between mass and volume, though
general relativity relates between them. You will sleep better if you think of electron
not as a point-like, but as of very small, much smaller than we can presently
probe experimentally. On the theory side, we might not be able to understand
electron's size until a theory of quantum gravity is developed. So, electron is probably
very complicated object but for all practical purposes its size can be ignored
and it can be regarded as point-like. This is a limitation of our present day knowledge.
The good news is, as you see, there are still many fundamental questions in physics
that need to be solved. You should take it as a motivation to learn today and be one
of whose who pushes the frontier tomorrow.