I have to call you out because the answer to this is in the docs. Space info can be found at game features -> space trading.
"A mission between a space center and a space station or between space stations will be counted as a single mission. Flying between space centers counts for 2 missions."
"If a maintenance unit is not available, the shuttle will age 5 times faster."
So, in short, each time a shuttle launches or lands at a space center, one mission is expended. Most missions you run will likely be between space centers. Count 2 missions for a normal run or 10 with no maintenance available.