Cannabis is different for everyone, so it is largely trial and error. That said, you should always error on the side of caution and start very slowly. The nice thing about vaping or smoking in general is that the full effects are felt within seconds to minutes, so I would suggest take one inhale and see how you feel, then another and so on. I am highly THC sensitive so one inhale for me may be enough but for my partner it may take 5-10 inhales to be sufficient. Unfortunately, there is no recipe for this that is one size fits all.
The other thing to be aware of is the % of THC in your vape pen cartridge. Is it 5%, 20%, 90%? The THC levels vary wildly and I just gave examples, this too influences your tolerance with your pen.
I would guess by your question that you are perhaps new to cannabis. Always error on the side of caution, always! Also, know that a high CBD tincture can help subdue an intense high if you should happen to get into a situation where you feel uncomfortable. Please know that if you feel uncomfortable, it will wear off, just drink a glass of water and go to bed!
I am not a doctor and this answer represents my opinion only. I hope this helps.
The best way to figure out your ideal dosage when vaping will ultimately come down to a trial-and-error process. We suggest starting with very low THC strains (like our Harlequin) and then slowly moving up to strains with more THC. This way, you'll never put yourself in a situation where you are "too high" and end up having a bad experience. We sell vaporizers and flower/concentrates (greendoorwest.com/collections/all-flowers) ideal for vaping (https://greendoorwest.com/collections/vaporizers-gear)