Maximum number of electrons a shell can accommodate is #"2n"^2#.
Third shell contains maximum #2 × (3)^2 = 18# electrons. The one given in the image is incorrect.
#underline(bb"Shell (n)" color(white)(.......) bb("Maximum electrons" ("2n"^2)))#
#color(white)(...)1 color(white)(..................................) 2#
#color(white)(...) 2 color(white)(..................................) 8#
#color(white)(...) 3 color(white)(.................................) 18#
#color(white)(...) 4 color(white)(................................) 32#
What you need to know is that when a shell is being filled there is no necessary requirement that inner shells should be completely filled.
Electrons are filled based on Aufbau principle. For e.g. when #3^"rd"# shell (which can accommodate maximum #18# electrons) is filled with #8# electrons we start filling #4^"th"# shell (which can accommodate maximum #32# electrons). When #4^"th"# shell is filled with #2# electrons we then start filling electrons in #3^"rd"# shell until it is completely filled. Note here that we have filled #2# electrons in #4^"th"# shell before completely filling #3^"rd"# shell.
For first #18# elements of periodic table electronic configuration is in the order #2,8,8#. For these elements when electron is filled in #n^"th"# shell #(n - 1)^"th"# shell should be completely filled.
#underline("Additional:")# #2,8,8,18,18,32# is difference in number of protons (or electrons) when we move from top to to bottom of same group. For e.g. differences in number of electrons of #"Na"# and #"Li"#, #"K"# and #"Na"#, #"Rb"# and #"K"#, #"Cs"# and #"Rb"#, #"Fr"# and #"Cs"# is #8,8,18,18,32# respectively.
