How much ram should my new MacBook Air be using when I only have a few safari tabs open?

macOS believes unused memory is wasted memory. You will see a lot of cached memory in Activity Monitor for this very reason. As long as the memory pressure gauge is not red or yellow, then you are likely Ok on memory for your workload. However, if the memory pressure gauge is green and you see a large GB range of Compressed Memory or Swap, then that can indicate you do not have enough memory for your workload.

I personally recommend getting at least 16GB of RAM. Web browsers can chew up the RAM very quickly these days depending on how many tabs/windows are open and how much junk those websites are loading for each page. Some people may be able to get by with just 8GB of RAM for checking email & limited web browsing and simple documents, but as soon as you start to do more work and keep more apps & windows/tabs open, the memory gets used up quickly.

It should use as much RAM as possible. You paid for 8GB RAM why would you not expect it to be used? If you get 16GB RAM, 16GB will be used. RAM is dynamic memory managed by the MacOS. Unused RAM is wasted RAM.

More RAM is always a better choice when purchasing a computer.

Something to add, some websites can chew up well over 1GB of RAM. I have come across one website that used up over 2GB!!! The ones that chew up a lot will be ones that can just continually scroll to reveal more content and if the content is animated media or constantly changing add boxes, the RAM usage can quickly increase.

With the availability of high speed internet, web designers are downloading more and more active content that needs space in the users computer.