2 Samuel 6:23

23 Therefore Michal the daughter of Saul had no child unto the day of her death.

2 Samuel 21:8

8 But the king took the two sons of Rizpah the daughter of Aiah, whom she bare unto Saul, Armoni and Mephibosheth; and the five sons of Michal the daughter of Saul, whom she brought up for Adriel the son of Barzillai the Meholathite:

Did Michal, who was at one point David's wife, die childless, or did she have 5 sons?

One resolution simply takes the text at face value - which says Michal "brought up" the five sons of her sister, Merab (1 Samuel 14:49), who was married to a man named Adriel (1 Samuel 18:17-19). After Merab presumably died, Michal raised them, with a man named Phalti (1 Samuel 25:44).

Another resolution is that Merab herself, not Michal, is named in some Masoretic, Septuagint (Greek Old Testament), and Syriac manuscripts (E.W. Bullinger), which would mean the proper reading should be Merab, and this only represents a mistake in textual criticism.