Number of optical isomers of lactic acid are

Only one chiral center. Hence two optical isomers are possible.

No. of optical isomer = $2^{\mathrm{n}}\left(\mathrm{where}\mathrm{n}=\mathrm{no}.\mathrm{of}\mathrm{chiral}\mathrm{carbon}\right)={\text{\hspace{0.17em}2}}^1=2.$