Short answer: less than you think, but still worth considering. Squarespace, WordPress, and Wix can all rank well if built correctly. What matters more is whether your platform supports the SEO fundamentals your photography website needs.
WordPress offers the most control over technical SEO and is easiest to customize for photography-specific markup. Squarespace and Wix handle basic SEO reasonably well out-of-the-box, but offer less flexibility for advanced optimizations like schema markup for local photography services.
The real issue isn't the platform—it's whether you're using it to optimize page speed, create location-specific pages (if you serve multiple areas), target photography-specific keywords, and build a proper internal linking structure. Many photographers use Squarespace successfully because it handles the fundamentals. Others struggle on WordPress because they ignore those same fundamentals.
If you're already on a platform and it's not limiting your ability to add title tags, meta descriptions, alt text to images, and structured data, switching platforms won't move the needle. Focus energy on content and optimization instead.