Yes, you can truly love more than one person at a time. I do, and I know many, many other people who do.
There's no magical light switch in your soul that makes you suddenly incapable of being in love with anyone else the moment you fall in love with someone. We tell cultural stories about how "true love" means one and only, but the reality of the human heart is more complicated than that.
You absurdly can be in love with more than one person at once. If you are monogamous, that's inconvenient, and it might mean you have to make a hard choice, but yes. You can truly love more than one person at the same time.