To collect a set number of surveys open the survey 'Detail' pop up by clicking on the detail button in the survey editor (circled below).

 

 

 

 

The following pop up will appear.  Enter the total number of required surveys in the text box titled; 'Sample Size' (circled below).

 

 

 

 

NOTE: If you are planning on using a 'Quota' question in your survey then the quotas you set must sum to this total 'Sample Size' number recorded here.  In principle this is common practice but the system will let you set quotas that sum to more than this sample size after you have launched the survey by using the settings in the ;Monitor Survey' control. More info on this subject here.