Usually, when you receive digests in your email, you can see 5 digests, and then there's a link to access others. If you choose the big digest option, you can receive up to 50 mentions in your email and link it to the rest of them.
You can activate the option while creating digests. If you select Mentions in email, you'll be able to activate it by checking the box Big Digests.
To learn more about Digests, check out our blog