You can use the pen until your run out of insulin
There is different sized pens and if for example it contains 300 units and you only inject 30 units the pen would contain "10" doses
Just follow the instructions that came with it to change the tip (sharp - don't ever reuse the "needle") and prep it for next use
If you google the "brand" of your pen you will likely find some video's on the product website that demonstrate "what to do"