It has been suggested that since the Hebrew word ELOHIM which appears in 1 Samuel 28:13 is plural, its proper referent in the text is spirits, not Samuel. This view is not impossible from an exegetical standpoint since the semantic range of ELOHIM certainly allows for this interpretation. However, I read ELOHIM in 1 Sam 28:13 as an intensive plural. That is, it appears to be talking about "a god" or "a divine being" instead of "gods" (i.e. spirits).
Commentators obviously give differing viewpoints on the construction but I found Henry Preserved Smith's take to be of interest (The International Critical Commentary on The Books of Samuel).
Smith argues that the "plural participle [in 1 Sam 28:13] would seem to indicate more than one ghostly figure. But only one is described in what follows, and we must suppose the agreement grammatical instead of logical. Similar instances of ELOHIM with a plural adjective are found Jos. 24:19 (E) Dt. 5:23 1 S[amuel]17:26, 36, etc" (A Critical and Exegetical Commentary on the Books of Samuel. Edinburgh: T & T Clark, 1969, page 241).
I tend to agree that the context indicates that only one spirit or "god" appeared. ELOHIM, as in the other places Smith cites, seems to appear with a plural verb in 1 Sam 28:13 for grammatical (i.e., agreement) and not logical reasons.