With Mail Merge for Gmail, you can send anywhere between 50 to 1500 email recipients per day and it will depend on the type of your Google account.
Also, the sending limit for Gmail is different from Mail merge because the latter sends via Google Scripts that have a fixed quota.
Know your Daily Sending Limit
Go to email quota checker and authorize with the Gmail / Google Account that you wish to use for sending email. It will show the maxium number of emails you can send in the current cycle. Google will reset your quota around 8 AM PST.
Email Quota for Gmail Account
If you have a @gmail account, you can email up to 50 email recipients per with the free edition of Mail Merge or up to 100 email recipients per day with the premium edition.
Email Quota for Google Apps Account
If you have Google Apps legacy account, where you don’t pay Google a monthly fee per user, your daily limit will be the same as those of free Gmail accounts.
If you have a Google Apps for Work account that is older than a few months, you can send up to 1500 emails per day. Please note that Google will only increase your daily email sending limit of Google Apps after a few billing cycles.
As per Google Support, any newly created Google Apps domains are subject to the consumer limit for the first billing cycle if they have six or more users, or several billing cycles if they have fewer uses.
Google Apps for Education, Government and Non-profits too have the higher 1500 limit.