We have found that there are 7 base units that are needed to derive every other units. In the SI system those 7 units are metre, kilogram, second, ampere, kelvin, candela and mole. You can read more about it [here](https://en.wikipedia.org/wiki/SI_base_unit
). However if you are careful you could choose other base units. For example the coulomb can be substituted to the ampere and the system is still complete.