I want to send mail to multiple people simultaneously in ERPNext, how to implement it?

What is the use case? please elaborate on it.

Otherwise, check this.

Like which shopkeepers have a particular product, we created a custom doctype and added the item in it and created a custom table in which we have to add the mail of every shopkeeper who has that item and send a message to all of them at once asking if you have this product or not and if yes, then at what price and we also want to mail more than 100 people at once

that for, you have create a workflow and some customization needed for email for this (send a message to all of them at once asking if you have this product or not and if yes)

If there are multiple email ids in the table then can we email them all at once

yes, you have to write a server logic.