i wanna buy a macbook air 128gb how much user space? do i really get in that device????

Welcome to the Apple Support Communities!

I understand you are considering purchasing a MacBook Air with 128GB of storage space. The amount of available storage space that you will have on the internal hard drive for data is dependent on the applications that you install in addition to the macOS High Sierra operating system which requires approximately 14.3GB. Depending on their size, you may need to store your large music, photo and video libraries on an external drive.
macOS High Sierra - Technical Specifications
MacBook Air (13-inch, 2017) - Technical Specifications​
Take care.

I just purchased this machine about a month ago. Now having storage issues as it says I have 94GB of system files. I have less than 20 GB of Apps and keep getting storage full popups. Cant even do a Time Machine backup. Is that normal amount of system files?