[I posted this answer to address a followup question that appears to have been subsequently deleted. I will leave my answer up as it does add a more complete details than the prior answer, which was correct albeit terse.]
You can mail multiple photos, but not an unlimited number. I assume to limit the overall email message size, as there could be limits on some mail gateways.
Open Photos app, select "Camera Roll" (or other album) from your albums. Touch the "Edit" button, and the screen heading should change to "Select Items." You can then select multiple photos, then touch "share" and select "Mail". If you don't see "Mail" as one of the options, then you have selected too many photos to attach to a single message. Try just two to see what it should look like. On my phone just now I was able to choose 5 and still have Mail as an option, but if I chose 6, then Mail was not an option.