Is it okay to design my book cover with standard license pictures?

I’m not exactly sure what you are trying to say but I think you are trying to license two to three pictures for a book cover that you are designing. Will all three images appear on the front cover? Adobe Stock’s standard license allows you to use licensed images perpetually but your print runs of the book can’t exceed 500,000. Secondly, as you indicate, because the book cover is not the main incentive to buy the book, as long as your print run (note this includes both print and digital editions) stays below 500,000 copies, you are good with a standard license. I believe this is the correct interpretation but be sure to look at the specific images and the license you purchase to be sure. Making a copyright mistake with your book cover can really cause a lot of problems!

Well, you seem to be well aware of the licensing terms. Indeed, if the cover is not the main reason to buy your book, you should be OK with a standard licence.

Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).