Often cfw pearls are sold predrilled on a temporary 15-16 inch strands. So probably you would want 1 1/2 strands minimum. Depending on the quality you purchase, there might be a few pearls you wouldn't want to use. Two full strands would give enough pearls for bracelet and earrings, too! Just the thoughts of a "lowly beader," who doesn't always purchase AAA pearls.
My knots average about 1mm each. So 50 knots would add about 2 inches. I can estimate fairly closely, but sometimes need to add or leave off a pearl at the end to get an exact length. Clasps vary quite a bit also, from about 1/2 inch to much longer.