We try to ensure that the number of testers you request actually complete each test by inviting the exact number you request first, and then slowly adding more invites as we see the total completions. Our algorithm tracks each test accordingly and will stop sending invites once the exact number is reached.
The main scenario where you would receive less testers than requested is if your targeting criteria (age, location, income, etc) or screening survey is too strict and limits how many testers are invited or accepted. Other scenarios include:
Complex test instructions or confusion from testers
Difficulty accessing your product or bugs that prevent testers from participating
Please note for bug tests: Testers who submit bugs but don't complete all tasks may still count in your tester total due to their time and commitment to try to participate and discovering bugs, even if those bugs prevented them from participating in all tasks.