So this is how you can change how many passenger spots there are for your tour on any specific day:

#1 Go to Manage Tours in Menu on top right of screen
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#2 Find the tour you want and click on EDIT

#3 Go to the "Schedule" tab
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#4 Click on the timeslot you would like to change

#5 Make sure you select "Only this time slot" if you want to change amount of spots just for that timeslot. If you want to change spots for entire schedule, select "All time slots in this schedule" but it be careful because this will change all timeslots in the schedule.
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#6 Now enter the new amount of spots in the "AVAILABLE SPOTS (PAX)" section.
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#7 Then scroll to bottom of the window pop-up and click on SAVE
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And that is it, you are done! 

May the force be with you :)