Key lime trees can survive planted in the ground fairly well as far north as USDA climate zone 9A but Austin and Central Texas are in 8B. See map below,
In order for your key lime tree to survive planted in the ground you must wrap it with an insulating material, such as a heavy blanket when below freezing temperatures are expected. Below 20 degrees the tree may not recover.
The nice thing about key limes, also known as Mexican limes is that they do well in a pot. Thornelss varieties can be brought indooors during the coldest winter days and returned to the patio for sunny winter days.
We keep our key lime in a large pot that is on rollers.
Choose a very large pot, on that is approximately two feet across and three feet high. These are available in a polystyrene material that is lightweight.
You can purchase rolling plant stands at Home Depot. This makes it easy to roll the plant out onto the balcony on sunny winter days. In summer it should stay outside all the time.
Key limes or Mexican Limes will survive outdoors in Central Texas, including Austin only if you protect them.
Keeping them is a pot is a good option and if you choose a large enough pot you will get just as many limes.
Key limes are self pruning, meaning the tree will find it’s own height so you don’t need to worry about trimming the limbs.