When looking for an answer to a different question, I came across this comic panel, showing the Hulk grabbing and shattering Thor's hammer:
How many times has Thor's hammer been destroyed?
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Mjolnir has been destroyed a total of six times in the comics.
The panel you have listed comes from Incredible Hulk #227 and is a dream sequence and so is not counted as an actual breaking event. (Thanks to @RDFozz in the comments)
Although an incredibly powerful weapon, Mjolnir is not indestructible. It has been damaged several times over the years:
- A force beam from the Destroyer slices it in two in Journey Into Mystery #119. Mjolnir was repaired in Journey Into Mystery #120.
- The Molecule Man dispels the atomic bonds between the hammer's molecules, vaporizing Mjolnir in Avengers #215. Mjolnir was restored in Avengers #216.
- The hammer shatters after channeling an incredible amount of energy at the Celestial Exitar in Thor #388 and was restored by the Celestials' replicoid in Thor #389
- Dark God Perrikus slices Mjolnir in half with a magical scythe in Thor Vol 2 #10. Mjolnir was restored in Thor Vol 2 #11
- The hammer is shattered when it collides with the uru weapons of Loki's followers, resulting in an atomic-scale explosion in Thor Vol 2 #80. Mjolnir wasn't seen again until Thor Vol 3 #1.
- The hammer was shattered by Bor in Thor #600, requiring Doctor Strange to reconstruct it by channeling Thor's Odinforce into its fragments.
Thanks to @Valorum's comment which led me to a different source for the information