This one is a great one for two birds but no more than that. If your budget is larger than I would go for something bigger. My budgie came with his wings clipped and they’ve since grown back. He’s not nearly as good of a flier as my other bird who was never clipped but they will learn. I will say that once his wings grew back, my budgie started trusting me and my wife wayyyyy more and it was super awesome.
Nope, way too small. 15X11X21 are the dimensions of a travel cage, it would be too small for even one budgie to live full-time. An absolute minimum cage size for two budgies would be something like this: https://www.amazon.com/Yaheetech-Stackable-Parakeets-Cockatiels-Lovebirds/dp/B0991RP7RT/ref=sr_1_9 if they get time to fly outside the cage as well.
The bigger a cage you can afford, the happier they will be. A popular one is https://www.amazon.com/Yaheetech-Wrought-Cockatiel-Cockatoo-Parakeet/dp/B079JW8YCQ/ref=sr_1_4