"In the 19th Edition of Remington’s Science and Practice of Pharmacy, published in 1995 (11), the recommended daily oral intake of Lugol 5% solution for I supplementation was between 0.1 and 0.3 ml. This time-tested Lugol solution has been available since 1829, when it was introduced by French physician Jean Lugol. The 5% Lugol solution contains 50 mg iodine and 100 mg potassium iodide per ml, with a total of 125 mg I/ml."
If you multiply through by 1000, then the above could read -- "The 5% Lugol solution contains 50 g iodine and 100 g potassium iodide per 1000 ml." I don't know whether 1000 ml of Lugol's solution weighs 1000 g, but there always seems to be that implication when you see that 50 g iodine divided by 1000 g solution = 5%.
Naming a product according to the percentage of one of the ingredients doesn't change the fact that another ingredient is involved which also gives benefit. One drop of 5% Lugol's = 2.5 mg iodine + 3.75 mg iodide = 6.25 mg iodine/iodide