You can send a mail to multiple recipients in salesforce. for that modify your code with the following,
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
mail.setToAddresses(new String[] { 'TestUser@salesforce.com' ,'abc@test.com'}); //add other emails here.
Please refer below link for more details. https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_calls_sendemail.htm
You can send a mail to multiple recipients in salesforce. for that modify your code with the following, Please refer below link for more details.
https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_calls_sendemail.htm
Regards
Ajvad Aju