The number of neighbors you can visit a day who you can collect items from and help depends on your reputation level. Starting from the first level, you can visit 4 neighbors a day and you can sponsor them once every 24 hours. You can increase your reputation level to get the opportunity to visit more neighbors by helping them. Each level up in your reputation adds one neighbor to the number of friends you can visit. At present, level 9 is the maximum reputation level.
Airport City: What do I need Control Tower for?
Airport City: What are Adventure Ratings?
Airport City: How do Runways work?
Airport City: How does a Terminal work and what are the sections for?
Airport City: What do I need a Ground Control for?
Airport City: How do I add friends?
Airport City: How can I launch a rocket with my neighbor?
Airport City: Cargo Cult
Airport City: Why do I need parking lots?
Airport City: What is a Stand?