The Death Star's weapon was made with kyber crystals, the same material used to make lightsabers.
How many lightsabers could be made with the quantity of kyber crystals that were used to create the Death Star I?
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The first Death Star contained multiple large kyber crystals. We can see what they look like in this still from the unfinished episode Star wars: Clone Wars - In Search of the Crystal
Assuming it has 8 in total (one for each energy beam) and that they're each as large as the one that Obi-Wan and Anakin intercepted (about 6ft x 4ft x 4ft), we can make some calculations.
A kyber crystal shard needs to be about 1 inch x 1/3 inch x 1/3 inch to use in a lightsaber or approximately 0.00000147 cubic meters. Assuming the larger crystals can be subdivided into individual crystals without losing much material, the answer is that a crystal that's 2.7 cubic meters could make around 1.8 million lightsabers.
Multiplied by eight, you could make something like 14 million lightsabers from the crystals used in the Death Star I.