If you've enabled both HTML and Text email formats for your mailings (Admin > System Settings > Supported Formats), your subscribers can choose whether they'd like to receive messages in HTML or plain text formats. HTML emails support graphics and text decorations while text emails display...well, text. Getting the counts for each group is just a click away.
Since HTML is widely supported and available on most mobile devices, the majority of subscribers choose to receive their emails in this format. However, knowing HTML vs. Text subscriber counts can be helpful in segmenting your groups.
To view yours, navigate to Subscribers > Reports > Subscribers by Email Format Type.
The breakdowns between subscribers receiving HTML format and text format are displayed. This gives you the insight into understanding what presentation preferences your subscribers exhibit.
If you find that you have a substantial percentage of subscribers reading your mailings in text, it would be worthwhile to reach out to them to find out why. With that information, you can make some decisions regarding your mailings' format and layout that might win these text format folks back to the HTML camp.