How many total electrons can be contained in the 4d sublevel?

2

6

10

14

The maximum number is 10

The s sublevel has just one orbital, so can contain 2 electrons max. The p sublevel has 3 orbitals, so can contain 6 electrons max. The d sublevel has 5 orbitals, so can contain 10 electrons max. And the 4 sublevel has 7 orbitals, so can contain 14 electrons max.

So, having this in mind, 10 electrons in total can be contained in the 4d sublevel.