This is the free version which limits the maximum recipients to 5. The full version has no limits: https://play.google.com/store/apps/details?id=com.tompierce.smsmerge
SMS Merge lets you merge contact information into your text messages. By using a placeholder such as #firstname, you can send the same message to many contacts, and each one will be personalized to the individual.
Currently supports the following data from your contact's details: #firstname - first/given name #lastname - last/family name #fullname - contacts full name #nickname - contacts nickname
To preview a message before sending, simply tap a name in the Recipients list.