I heard that king David had 18 wives.
Approximately how many wives did king Shaul have?
The Book of Samuel only tells us of King Shaul having had 1 wife and 1 concubine.
Shaul's wife (I Samuel 14:50):
וְשֵׁם֙ אֵ֣שֶׁת שָׁא֔וּל אֲחִינֹ֖עַם בַּת־אֲחִימָ֑עַץ וְשֵׁ֤ם שַׂר־צְבָאוֹ֙ אֲבִינֵ֔ר בֶּן־נֵ֖ר דּ֥וֹד שָׁאֽוּל׃
The name of Saul’s wife was Ahinoam daughter of Ahimaaz; and the name of his army commander was Abiner son of Saul’s uncle Ner.
The woman King Shaul took as a concubine (II Samuel 3:7):
וּלְשָׁא֣וּל פִּלֶ֔גֶשׁ וּשְׁמָ֖הּ רִצְפָּ֣ה בַת־אַיָּ֑ה וַיֹּ֙אמֶר֙ אֶל־אַבְנֵ֔ר מַדּ֥וּעַ בָּ֖אתָה אֶל־פִּילֶ֥גֶשׁ אָבִֽי׃
Now Saul had a concubine named Rizpah, daughter of Aiah; and [Ish-bosheth] said to Abner, “Why have you lain with my father’s concubine?”