A molecule is non-polar if its net dipole moment is zero.

Check each molecule:

1. $\mathrm{HF}$: heteronuclear diatomic molecule, so it is polar.
2. $\mathrm{H_2O}$: bent shape, dipoles do not cancel, so polar.
3. $\mathrm{SO_2}$: bent shape, so polar.
4. $\mathrm{H_2}$: homonuclear diatomic molecule, so non-polar.
5. $\mathrm{CO_2}$: linear and symmetric, bond dipoles cancel, so non-polar.
6. $\mathrm{CH_4}$: tetrahedral and symmetric, so non-polar.
7. $\mathrm{NH_3}$: trigonal pyramidal, so polar.
8. $\mathrm{HCl}$: heteronuclear diatomic molecule, so polar.
9. $\mathrm{CHCl_3}$: tetrahedral but unsymmetrical ($3$ Cl and $1$ H), so polar.
10. $\mathrm{BF_3}$: trigonal planar and symmetric, bond dipoles cancel, so non-polar.

Therefore, the non-polar molecules are: $\mathrm{H_2},\ \mathrm{CO_2},\ \mathrm{CH_4},\ \mathrm{BF_3}$

Number of non-polar molecules: $4$