All the given species undergo dimer formation. Find out the number of species for which dimerisation; does not require vacant orbital.

${\mathrm{AlCl}}_3,{\mathrm{BH}}_3,{\mathrm{ClO}}_3,{\mathrm{ICl}}_3,{\mathrm{NO}}_2,{\mathrm{BeCl}}_2$

${\mathrm{AlCl}}_3,{\mathrm{BH}}_3,{\mathrm{ClO}}_3,{\mathrm{ICl}}_3,{\mathrm{NO}}_2,{\mathrm{BeCl}}_2$

All the unstable and e deficient (hypo valent) species if they tend to dimerise then they use the vacant orbital of it. For example ${\mathrm{BH}}_3,{\mathrm{BeCl}}_2$

Note the odd e species like  ${\mathrm{ClO}}_3,{\mathrm{NO}}_2$ do not uses vacant orbital they use the odd e' for pairing and forming dimer.

${\mathrm{ClO}}_3$: dimerize due to covalent bond

${\mathrm{NO}}_2$ dimerize due to covalent bond

Thus, ${\mathrm{ClO}}_3\mathrm{\&}{\mathrm{NO}}_2$ donot uses the vacant orbital they use the odd  $e^{-}$ (unpaired) species