When I came to this world with a dying body, I had never thought that I would have a huge amount of mana in my body.
Actually, I didn't know I had a lot of it until recently. If you knew me, you would know that I was very bad at self-assessment. I mean...I knew how much mana I needed to do this or that, instinctively. But I didn't really know how much mana I had compared to other people.
But then again, it wasn't like there was a measurement tool for counting someone's mana quantity. There was no numerical indication or fancy concept like first circle and five circles for a druid. In the first place, we needed mana to survive only, but we could use the mana in the environment for other things.
So...yeah. That was my justification for not knowing how much mana I had.