How many photos can I include in one message

Your ISP also has limits on email message attachment sizes. These are all anti spam measures. You should ask your internet service provider what their maximums are and also ask your cell provider. They publish those usually within the contract terms. You may need to send several emails with some photos in each one.

You can always bundle them up, i.e. compress them, and send then with Mail's "Mail Drop" feature.

Mail Drop limits - Apple Supporthttps://support.apple.com

Add an email attachment in Mail on iCloud.com - Apple Support

Guess, as many as your Mobile Phone provider Mobile Data plan allows.