I was planning to create a realm on Minecraft Bedrock so my friends could play on my world when I'm out and about.
How much data volume does it take to play mobile data on this realm?
Approximately 100 - 250 MB per hour. Depending on how much you walk in the world (load new chunks of course, data needs) etc.
But that only counts for the PC version - maybe it was optimized for the phone, then vlt. Even less.
https://gaming.stackexchange.com/questions/209413/how-much-network-data-does-minecraft-normally-use