There are no settings in the admin GUI or something where you can change the number. There was a design decision to have a specific number of icons depending on the width of the window.
While this doesn’t help much, I can show you where the setting is made in the code, but I have to remind you, that it is usually better to leave the code untouched and rather open a feature request.
While we are stubborn and doing it anyway:
../nextcloud/core/js # less js.js
Search for “minAppsDesktop” and you will find:
var minAppsDesktop = 8;
Increasing that value will increase the number of icons in the app bar - on the desktop (doesn’t affect mobile).