The 4d subshell can hold a maximum of 5 orbitals or 10 electrons. Any given 4d orbital can hold a maximum of 2 electrons though.
Explanation:
Since 1 orbital can hold a maximum of 2 electrons, and there are 5 orbitals in the 4d subshell, it can be concluded that there are a maximum of 10 electrons allowed. Coming back to your question, any given 4d orbital can hold a maximum of 2 electrons.