Hey, @rhonda8! Welcome to the Community and thanks for starting this topic.
Currently, applying Captcha on customer account logins is not an available feature. However, our developers are looking at options to make it available, and will share an update in the Shopify Changelog when/if it happens.
In the meantime, I am happy to pass along your feedback regarding this to them. Could you tell me more about what you are currently experiencing with customer logins that would benefit from having a Captcha?