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

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Stage 1. Dissociation of the substrate, forming the planar carbocation
 
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Stage 2. Attack of the nucleophile on both sides giving rise to the formation of two enantiomers in equal proportion (racemic mixture).
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The products formed are enantiomers and are obtained in equal amounts, since the two faces of the carbocation are indistinguishable for water.