public class DemoBlockEntity extends BlockEntity {

    // Store the current value of the number
    private int number = 0;
   
    public DemoBlockEntity(BlockPos pos, BlockState state) {
        super(TutorialBlockEntityTypes.DEMO_BLOCK, pos, state);
    }
   
    // Serialize the BlockEntity
    @Override
    public void writeNbt(NbtCompound nbt, RegistryWrapper.WrapperLookup wrapper) {
        // Save the current value of the number to the nbt
        nbt.putInt("number", number);

        super.writeNbt(nbt, wrapper);
    }
}