Thank you @Ciprian Rusen for your feedback. I have captured it for the team that owns the new tab experience. This may be a poor substitute, but in the meantime, you can add more than 8 sites to the OnStartup behavior: edge://settings/onStartup
Are there any plans this feature is going to be implemented any time soon? I'm wondering how dev-team come up with 8 tabs being sufficient. Perhaps 10 years ago it would ok to have only 8 tabs but in my opinion it's not sufficient in modern nowadays browsers. Installing extension is not an option because it replaces some of the new build in features when opening a new tab.