Informal:
* A constellation of stars
* A galaxy of celebrities
* A gaggle of stars
* A bevy of beauties (if referring mainly to female celebrities)
* A crowd of VIPs
* A throng of paparazzi (if referring to celebrities being surrounded by paparazzi)
More formal/specific:
* A cast of characters (if referring to celebrities in a particular industry, like actors)
* A roster of stars
* A pantheon of icons (if referring to very famous, influential celebrities)
* A collective of influencers (if referring to celebrities who heavily impact their audiences)
Ultimately, the best collective noun depends on the context and what you want to emphasize about the celebrities in question.