The number of stereoisomers formed in a reaction of $\left(\pm \right)\mathrm{PhC}\left(=\mathrm{O}\right)\mathrm{C}\left(\mathrm{OH}\right)\left(\mathrm{CN}\right)\mathrm{Ph}$ with $\mathrm{HCN}$ is

Aldehydes and ketones undergo reaction with HCN to produce cyanohydrins. The reaction progresses very slowly by using pure hydrogen cyanide. Hence, base as a catalyst helps to speed up the reaction. This is an example of nucleophilic addition reaction.

here we are given racemic mixture of optically active compound. The attacking nucleophile can attack on carbonyl carbon from either side forming a new chiral carbon. So, in product there will be two chiral carbon but it is symmetrical. So, number of stereoisomers will be 3 only.