Regarding how many migrated from Philips Hue to the ZHA (Zigbee Home Autoamtion) integration you can make guesses based from analytics statistics → https://analytics.home-assistant.io/#integrations
Note that those analytics/statistics are only collected from those who choose to “opt-in” and are estimated to only be a fifth of all Home Assistant actual userbase → https://community.home-assistant.io/t/home-assistant-founders-believe-there-is-currently-around-50-000-installations-of-zha-integration-what-do-you-zigbee-users-in-the-community-think-about-those-statistics/357305/