Unless its a state compliant (10 round mag ) it comes with 1 30 round magazine check the specific model here https://ruger.com/products/ar556/models.html

Most AR 15 models will include (1)-30 round Magazine.

Hi Tim, you did not specify an exact model. They come with one magazine, and most model come with a 30rd magazine, except for the state compliant models.
30rd model here budsgunshop.com/product_info.php/products_id/95081/ruger+ar-556+16.1+223+remington+5.56+nato+ar15+semi+auto+rifle ,
Also, these will accept any standard AR15 magazine budsgunshop.com/product_info.php/products_id/1122/magpul+pmag+gen2+moe+black+5.56+30rd+magazine ,
Stay safe & have a great day!

Hi Tim and welcome. (1) Magazine is included. 30rd magazines available here: budsgunshop.com/product_info.php/products_id/1177/magpul+mag557-blk+ar-15+pmag+gen+m3+30rd+.223rem+5.56nato+black ------------------- Cheers and stay safe!