1 define the term "visitors", for example, does it only refer to passengers, or airport working staff also need to be considered.
2 segment visitors into three groups (by their objective of visiting): A passengers B friends/ family that come together with passengers C airport working staff;
3 group A guesstimation: from supply side: number of flight that takes on/land in one day * average number of seats per flight * attendance rate (first and third factor could be further segmented according to flight take-off time); or from demand side (harder than supply side): people who leave(tourist leave+citizen leave) + people who arrive(visitor+citizen come home), for citizen part, we could use total number of citizens * percentage, and could further segment with age/ career...
4 group B guesstimation: segment group A's number by age, then for each group, use the "accompany number * passenger number" method
5 group C guesstimation: independant of group A and B; list out departments in airport, and then guesstimate how much staff there will be in each of them
6 add up three numbers: A+B+C; then it will be the result