Hi Nithya Santhoshini,
As Ronald Catersels rightly pointed out, a mail can be sent to multiple recipients by using a comma separated list.
Situation 1 : When the Emails are a static, predefined emails such as “firstname.lastname@example.org”, “email@example.com” and “firstname.lastname@example.org”, then in this case, you can use the comma separated list as shown below
Situation 2: When the emails are dynamic and are defined from attribute values, (as in your case from $Instrument/DistrictManager), then you can loop through the $Instrument List and create the To String. Something as shown below:
Use the comma to seperate those. So like: email@example.com, firstname.lastname@example.org
Try to use the common values