The best guidance is taken from your analytics. If you know your peak hourly visits, the following calculation will determine the number of virtual users needed for your test(s):
V = P / (60/D)
V = The number of Virtual Users needed
P = The number of Visitors in your peak hour
D = The average duration in minutes of each visit
Additionally, you may want to increase V by a percentage that represents growth. Example: if your are using analytics that are from last year’s peak hour during the holidays, and you feel that there will be a 20 percent increase this year, increase V by 20 percent.
V = (P / (60/D) ) + .2 (P / (60/D) )