I'm using SharePoint Online modern experience.
When entering multiple users in Person or Group column, how many users can I enter?
If you don't set "All multiple slections" to Yes, you can only add one user to each field. If you set it to Yes, no evidence or documentation indicates that there is a limit of users or groups can be added to a Person or Group column. I'm afraid you would need to test it on your own to determine the maximum number (if there is), and I don't think you would reach that number if it exists in real world scenarios, because if you were to add a large number of users to this column, it would be a better idea to add these users to a group and add this group in this column. A user can belong to 5,000 groups per site (site collection), and each group can have up to 5,000 users. You can have up to 10,000 groups per site (site collection).