This actually varies for each knot used when making a necklace. It is usual to use about 3 beads per 10 centimetres of necklace, unless your preference is to use more. Here is a chart to indicate what the usual number of beads should be:
20-30 CM collar = 8 beads
30-40 CM collar = 10 beads
40-50 CM collar = 12 beads
50-60 CM necklace = 15 beads
60-70 CM collar = 18 beads