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The number of bookings for an event is not stored as a meta value for the event. There is a meta_key for the event_id (to map a post_id to an event_id). The bookings are stored in the wp_em_bookings table. Each entry in the table has an event_id. You can get the number of bookings for an event by counting the number of bookings with a booking_status equal to 1 for the particular event_id.
Here are the post meta keys used by events manager for events:
_event_approvals_count
_event_start_date
_event_start_local
_event_start
_event_end
_event_end_date
_event_end_local
_event_id
_event_owner_anonymous
_event_owner_email
_event_owner_name
_event_rsvp
_location_id
_recurrence_id
_start_ts
_end_ts
In addition there may be post meta keys for custom attributes.