I assume you are referring to romantic love, or what people often refer to as "true love.". Yes, I absolutely believe it is possible to love more than one person in a lifetime. I believe many relationships have the potential to become "true love" - if both participants choose it. (Of course there needs to be some degree of compatibility as well.)
I would say that I've been in love 3 times in my life. The first two times, I was not ready to make the choice to love that person for the rest of my life. The third time, I was, and I did, and it makes all the difference. However, if my husband were to pass away (heaven forbid!!!!!), I think that eventually, after grieving for him and healing, I would be able to love another and remarry. I suppose the same if we ever divorced, but it'd have to be something pretty major for us to separate. I think we both would make every effort to heal our relationship and stay together. But, it happens, and I won't be so arrogant after 1 year of marriage to say that it could never happen to us.