Here I assume you have only the basic periodic table which doesn't mention electron configurations.
1s2 2s2 2p5
This reads that there are 2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 5 electrons in the 2p subshell. This is a total of 2+2+5 = 9 electrons. So for a neutral atom, we must have Z=9.