first of all we have to calculate iteration time in
vugenbased on thatwe have to set the scenario ramp up
,duration, rampdown.
in scenario creation we have to define
for ex in vugen for 1 iteration the time is 10 sec (login
2sec,action part of script6sec,lout of script2sec)
based on this we have to intialialise the vusers on to the
server like this
first 2 vusers starts when scenario begins then rest 18
users enter in following manner i.e
1)intialise 2 usersforevery 4 sec& runfor12 sce &rampdown 
from the server after duration.this is scenario setting.