Its does not possible to select several time-slots (possible to select only one), if you are using Booking Calendar Free version of in paid versions using range-time shortcode:

Select time-slot: [select rangetime "10:00 - 12:00" "12:00- 14:00" "14:00 - 16:00" "16:00 - 18:00" "18:00 - 20:00"]

Instead of time-slots selection you can use start time and end time selections or start time and time duration selections shortcodes at the Booking > Settings > Form page in paid versions of Booking Calendar. So you can book several consecutive hours during specific day.

1) Star time and duration of time shortcodes, for ability to book several consecutive hours during a day:

Start time: [select starttime "10:00" "11:00" "12:00" "13:00" "14:00" "15:00" "16:00" "18:00"]
Time Duration: [select durationtime "15min@@00:15" "30 min@@00:30" "45 min@@00:45" "1 hour@@01:00" "1 hour 30 min@@01:30"]

2) Or you can use start time and end time selections or time entering shortcodes:

 Start time: [select starttime "10:00" "11:00" "12:00" "13:00" "14:00" "15:00" "16:00" "18:00"]
 End time: [select endtime "11:00" "12:00" "13:00" "14:00" "15:00" "16:00" "19:00"]

You can configure the time-slots selections on the Booking > Settings > Form page. Please use the "shortcode generator" on the right side of that page for the correct generation of the shortcode.
Please watch more in this video: https://wpbookingcalendar.com/help/booking-calendar-video-timeslots/

You can test the bookings for the specific times in action on the live demo of Business Small version (which is configured by default for the bookings of specific time-slots) here: https://bs.wpbookingcalendar.com/
Other live demo you will need to reconfigure.