Welcome aboard,
DGM.

Always freely ask whatever questions & address any thoughts that pop into your mind -- you'll likely find you're wiser than you presently believe.
Most people in the world will say that monogamy is easier, not because it's actually
easier, but rather
familiar. After a few tries, almost everyone knows that the chances of the glorious Romantic happily-ever-after couple are rather low, but nevertheless they keep secretly hoping for (if not actively chasing) that ideal. The errors, the unsuccesses, the failures, the disasters... well, most of us have seen plenty of those as well, so we just sort of accept a certain degree of carnage & talk ourselves into believing it can't happen to us, whistling as we walk past the graveyard.
Nonmonogamy seems like a LOT more work simply because those of us who venture into it find ourselves NOT surrounded by the stereotypic situations we grew up seeing all around us -- as well as bombarded by in movies, TV shows, literature high & low, popular songs, famous works of art, even billboards & magazine ads.
In my experience, maintaining multiple happy intimate relationships is LESS work than always needing to "color inside the lines" of Monogamism & do stuff only in the
approved manner (without ever being presented with a nice LIST of those expectations let alone detailed explanations of HOW to fulfill them properly) & then there's the "keeping up appearances" stuff that feels somehow shameful even after it dawns on you that MOST people are faking their way through life.
For nonmonogamy, there's more overall effort required, at least at first: there IS a learning curve involved. And there does have to be a little ongoing effort put into not being constantly blatant in public, "not panicking the sheep," because a crazed herd can trample the heck out of you without a speck of bad intent.
And then you might turn around & realize that having multiple lovers (AND a social network of similarly inclined people) takes maintenance & upkeep, but really isn't all so difficult.
What are my chances of having more than one poly relationship?
About as high as you WANT them to be. You're mobile, & living in a populous region not dominated by self-righteous nutballs. You have (as it turns out) friends who at least
claim to be experienced in polyamory, a boon most people who show up here don't have -- this offers you tremendous opportunity to learn more about living polyamorously.
You will likely make mistakes; there may be outright failures. So long as you approach situations honestly AND work to clean up any errors AND continue to learn from misstep & success alike, you'll probably do fine.
Your specific situation? I'd recommend taking your time getting involved with your friend. You don't sound at all desperate to attach yourself to someone.
Getting intimate with a friend means redefining that friendship, whether the dyad is a roaring success or abject failure or ANYTHING between. Your background is monogamy, & learning how to do that & be comfortable requires you to have experience. In my life, I can have sex (once or occasionally) with a good friend & not have our friendship clouded up by Romantic nonsense; as well, I can turn a wildly sexual connection into longterm affection. Those skills took practice: wasn't easy the first time, a little difficult the next, briefly awkward the one after...
Hang out with your friends. Ask an occasional question, sure, but mostly just get a feel for "being poly," for living with the changed worldview. Watch how they act in public. Find out how they interact with other polyfolk.
Rather than doing the Monogamist thing -- leaping headfirst into a new dyad & sorting out the details later -- put effort into simply
dating your friend. Sure, you "know her very well"... except that
you DON'T: you have now been faced with a person who is much more complicated. You need to get to know THAT person, from the beginning, even though she looks & sounds & smells
very familiar. An intimate relationship isn't like a house where you can just tack on another floor; to continue the analogy, the foundation won't take it, so if you truly want the space, you need to either tear it down to rebar OR relocate to a more suitable domicile.
Now that you know your friends are poly, you WILL see their relationship differently, including all your memories of them. Now that you know your friend is attracted to you (& not dishonest about it to her partner), you WILL see her in a different light, & wonder at what point she became someone other than you thought.
For now? Date, hang out, adapt to the notion, take your time.