The SN1 reaction proceeds through a planar carbocation, which is attacked by the nucleophile on both sides, giving rise to a mixture of stereoisomers.

stereochemistry-sn1

Stage 1. Dissociation of the substrate, forming the planar carbocation

stereochemistry-sn1

Stage 2 .
Attack of the nucleophile on both sides giving rise to the formation of two enantiomers in equal proportion (racemic mixture).

stereochemistry-sn1

The products formed are enantiomers and are obtained in equal amounts, since the two faces of the carbocation are indistinguishable for water.