How many emails will be send using trigger per execution?
You can send as many as you need to, as long as you don't violate any of the governor limits. There is no specific limit to the number of emails that can be sent per execution. You are limited by a daily limit for non-user recipients, a per-transaction limit of 10 Messaging.sendEmail calls, and a maximum of 100 emails per Messaging.SingleMessage. It is conceptually possible to send 10,000+ emails in a single trigger execution if you design the code properly.