I think saying love is infinite is an exaggeration (I don't think you could possibly love everyone on the planet as a partner, for instance) but I do agree that its limit have to do with time, mostly.
I also think it depends on the people. I have seen people have only one child because they don't believe they'd be able to love two children as intensely as they love one (not that it's the only reason to have a single child, obviously!) and others who find they love their first child more when they have a second one. I've seen people who have only one or two friends and believe that if you have more friends, then they can't be really good friends, and others who have more friends and would feel like an island if they only had one good friend.
I think you can be "mono" in several forms of love. The only thing is that in our society, it's normal to be "poly" for some things (friends, family) and normal to be "mono" for some others (romantic relationships).
However, different people will be fundamentally one or the other in all of these types of love, I believe.
You can be thinking lovingly of several partners at once. You can also be in love as a general state, and not feel that you're in love with A specifically or B specifically. You're in love, that's your state of mind, and there are a number of people who contributed to that state (for instance A and B). Sometimes, being in love can feel more similar to being happy, and in this case you wouldn't say "who is making you happy? It can't be more than one person". Well it can, if they all contribute to it.
As for time, you can spend time with more than one person at a time. And giving your undivided attention? I'm sorry but it's never really the case with me. I'll always be thinking about things at the very least, and that takes some of my attention. During a date, I might be enjoying the food, and that takes some of my attention. It's even more obvious during a movie, and yet I might be holding hands at the same time and enjoying that.
I think people who think with that mono perspective tend to forget that you already give some of your time and/or attention to other things and people. You spend time at work, you spend time sleeping, you spend time enjoying your hobbies. You spend time with your family, with your friends, or on an Internet forum. It's quite possible to be mono and have much less time and attention to give your one partner than a poly person does with each of theirs.
And then sometimes there is a quality aspect to it as well. When you're together, especially for a long time, not all time spent together will be quality time. It could be spent doing the laundry while your partner is in the next room, or things like that. Yet when you have more partner, it's possible that the quality time is simply condensed. You spend less time with each partner, but it's also much more intense than it used to be. Things like that.
Ultimately, it will depend on the situation, the people involved, the type of relationship, but it's not as easy as "dividing" the amount of love by the amount of partners.