1 mol of any substance is made of 6.022 x 10²³ units, these units could be atoms making up elements or molecules making up a compound.
In this case. 1 mol of Li is made of 6.022 x 10²³ atoms of Li
The molar mass of Li is 6.94 g/mol
Therefore mass of 1 mol of Li is 6.94 g
In 6.94 g of Li there are - 6.022 x 10²³ atoms
Then in 766.3 g of Li there are - 6.022 x 10²³ atoms / 6.94 g x 766.3 g = 665 x 10²³
There are 6.65 x 10^25 atoms of Li