The email function only sends messages to workers who are qualified and eligible to take your HIT.
Each time you use the "Email Included Workers" function after launching your study, CloudResearch only emails workers who are qualified and eligible to take your study. If you just launched your study to 200 workers but CloudResearch shows emails were sent to 197, it may be because three people found your study and accepted it between the time you launched the study and sent the emails. Once a worker accepts your HIT they are no longer qualified (because they are in the middle of taking it or they took it already), and thus, will not be e-mailed.