With Mail Merge for Gmail, you can send anywhere between 250 to 1500 email recipients per day and your daily limit will depend on the type of your Google account. Google will reset your quota around 1 PM PST.
Mail Merge Quota for Gmail Accounts
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 250 email recipients per day with the premium edition.
Mail Merge Quota for G Suite (Google Apps)
If you have a Google Apps legacy account, where you don’t pay Google a monthly subscription fee per user, your maximum daily limit will be the same as those of free Gmail accounts.
If you have a paid G Suite account (Google Apps for Work) that is older than a few months, you can send up to 1500 emails per day. GSuite for Education, Government, and Nonprofits also have the higher 1500 limit provided you have upgraded to the premium edition of Mail Merge.
I’ve G Suite for Work but my limit is not increased yet?
If you have a new G Suite account, or if you have recently upgraded from free Google Apps to paid GSuite for Work account, Google will only increase your daily email sending limit after a few billing cycles.
Google Support says that 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.
I could only send half emails and my quota was exhausted?
Please note that the daily email quota for Gmail Mail Merge is total recipients per day. Thus if you send 5 email to 10 addresses (to, cc, or bcc), Gmail will count it as 50 recipients.
I can send more emails with the Gmail website. Why?
The sending limit for Gmail is different from Mail merge because the latter sends via Gmail APIs that have a different sending quota.
How do I check my Mail Merge Sending Limit
Go to email quota checker and authorize the Google script with the Gmail / Google Account that you wish to use for sending emails. It will show the maximum number of emails you can send in the current cycle.