Unfortunately I noticed that many companies started putting limits back on a few years ago. We have two options at my house: Mediacom and Century Link. Mediacom goes up to 150Mbps for 2000GB, which is their maximum package. Century Link only offers roughly 10Mbps DSL in my neighborhood, although they've been building out 1 Gigabit fiber in parts of town. Not sure if it's unlimited. When I had Charter a few years ago, it was unlimited but about 40% slower, and I can't get it at this address. My Grandpa has AT&T DSL, as that's his only option, and it's limited to either 250GB or 350GB. But it's so slow he could never hit that. I'm on Mediacom's 100/10Mbps plan, but in reality I actually get a better 110/20Mbps. I'm capped at 999GB, and I can't even upgrade to the 2000GB plan because they don't offer 150Mbps in my area. I asked. But we never hit 1000GB anyway, despite all the streaming and cloud stuff going on in our household. So some people might be able to upgrade, but pay a lot, and others like me have no option to.
My message was just a warning to those on limited plans, as there are lot of them out there who never come close to hitting their limit that might this one time. This first iCloud storage sync could make you get hit with overage charges—as well as any subsequent syncs to brand-new Macs that you purchase down the road. If you're loading up a 1 or 2TB iCloud plan, be careful!