How do I sign in to my MacBook Air when some keys are not working?

Use an external USB keyboard.

Then make sure you backup your stuff to an external drive so it isn't at risk of being lost when you take the computer in for service. Back up your Mac with Time Machine - Apple Support

Get your Mac repaired and see how much will it cost: Mac Repair & Service - Apple Support 

You can make a Genius Bar appointment for hardware/software service and support using this link: Contact Apple Support - Apple Support

Call Customer Support (800) MY–APPLE (800–692–7753) or on line /getsupport.apple.com/ 

or call AppleCare Support at 1-800-APLCARE (800-275-2273) 

Crombieas wrote:

How do I sign in on my mac as some key are not working?

[Re-Titled by Moderator]

If those keys are 789uiojklm, see

Fix for 789uiojkl keys or numeric keypad … - Apple Community