Scams (e-mail, text messages, and phone calls) are getting very good at closely imitating true Apple communications. Always be cautious. Here are some guidelines:
Identifying legitimate emails from the App Store and iTunes Store - Identify legitimate emails from the App Store or iTunes Store - Apple Support
Recognize and avoid phishing messages, phony support calls, and other scams - Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support
Apple 'How to identify, avoid, and report phishing' video--> https://youtu.be/SR3Z3fXXjfw
When in doubt, try to check the service directly with Apple using an interface you know is talking to Apple, not by any link in an email. Click on this link to read how to 'Manage your iCloud storage' --> Manage your iCloud storage - Apple Support - including seeing how much storage you have