Depends on which connector you are using.
The Send an email notification (v3) will send the email from Microsoft PowerApps and Flow. It can only send 100 emails every 24 hours.
The Send an email (v2) will send the email from you as the creator of the flow. It can send 300 emails every 60 seconds.
there is no single way to send out the bulk emails. You need to break your contacts into maximum group of 300 and wait 1 minute to send out email notifications to next group.
So inshort you need to run the flow for 233 minutes (70,000/300) minimum + execution time of the flow with a lot of supervision.
There are better solutions out there to send bulk emails.
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Thanks,
Rahber