The box at the top of the comms area will show you how many sms or emails will be sent with your current contact selection. This is to provide sight of which may be the best contact option.
For example if you have 20 contacts selected but only 10 of them have email addresses recorded however all 20 have mobile numbers you would see quickly from the top box that 20 SMS would be sent but only 10 emails. Therefore you may decide to send an SMS over an email.
You will only send the chosen method of communication, it won't send both a text and an email.