Google Photos use Google Drive Storage. At this time it has 15 GB for free. It's worth to say that ...

You can choose between 2 storage sizes to back up your photos and videos to your Google Photos library.

High quality

• Unlimited free storage
• Regular cameras: Recommended for phones or point-and-shoot cameras that are 16 megapixels (MP) or less.
• Uses: Good for typical printing and sharing.
• Size: Save high-quality photos and videos while reducing size.

Original

• Limited free storage: Uses your Google Account's 15 GB of free storage. Check my storage.

• DSLR cameras: Recommended if you take photos with a DSLR camera and want to maintain the exact original quality.

• Uses: Recommended for printing large banners or to store your original files.

• Size: Store your photos and videos exactly as you captured them.

References
Choose a storage size - Google Photos Help