Lifting weight is not the same as holding it
A standing desk gives you two different numbers to think about, even when the listing prints only one. There is the weight the frame can hold while it sits still, and there is the weight its motors can raise and lower. Those are not the same job, and the figure makers put front and centre is usually the first one. FlexiSpot's own spec for the E7 frame, for instance, calls its capacity a "payload static" of 125 kg, and the word that matters there is static: it is measured with the desk standing still and the load spread evenly across the top.
A desk that moves has the harder task. Raising a load against gravity, and doing it smoothly through the whole height range, asks more of the legs than holding that same load in one place, and it asks the most when the weight is bunched at one end rather than spread out. So read a static or maximum figure as a ceiling rather than a target. Load a desk to the number on the box and you have left the motors nothing in hand for the day you add a second monitor or a heavier screen.
Add up what will actually sit on top
The way to use the number is to work out your own load and hold it against the rating, rather than trusting that a desk quoting a big figure will swallow anything. Add up the things that live on the surface: the desktop itself, if the maker's figure is for the bare frame, then your monitors, a monitor arm, a laptop, a dock and a lamp. Each of those prints its own weight on its own listing, so you are adding real figures rather than guessing at a total.
It is worth checking the limits of the smaller things in that pile too, because the desk is not the only part with a rating. A monitor arm has a much lower ceiling of its own: the arm in our monitor arm guide is rated to hold 9 kg, so a heavy or oversized screen can be too much for the arm long before it troubles the desk. Read each component's number the same way you read the desk's, and let the strictest limit in the chain set what you can mount.
One motor or two, and where the load sits
Where a second motor earns its keep is less about the total weight and more about how that weight is spread. A dual-motor desk drives one motor in each leg, so both ends keep pace when the load leans to one side, the common case where an arm and a screen hang off the left of the desk and the right carries little. A single motor turns both legs from a shared drive bar, which costs less and copes fine when the load is balanced. Our small home office desk guide works through single against dual for a tight room in its FAQ, so the short version here is this: size up the weight you need to carry before you worry about the motor count.
Getting the desk to the right height once it is carrying all that is its own task, and our guide on how to set a standing desk's height goes through it.
FAQ
Q: Does the desktop itself count towards the weight capacity? A: Check how the maker states the figure. A frame-only desk quotes what its legs can carry, so the top you bolt on is part of the load, and a heavy solid-wood board eats into the number before you put anything on it. A complete desk's figure often already allows for its own top, but the listing does not always spell that out, so if you are close to the limit it is worth asking the seller rather than assuming.
Q: My desk lists one weight figure. Is that the lifting capacity or the static one? A: Often the listing does not say, and a single number is usually a maximum or static load rather than a separate lifting figure. Treat it as the most the desk should ever carry, not the weight it will happily move all day, and keep your real load comfortably under it. If a maker does print two figures, a static maximum and a lifting capacity, the lifting one is what governs a desk that moves.
Q: How much room should I leave under the rating? A: Enough that a normal day never reaches it. There is no official margin to quote, so rather than reach for a percentage, add up your real load, compare it to the maker's figure and leave headroom for the things you tend to add later, a second screen or a dock, without going back to the spec sheet. A desk sitting well below its rating is also asking less of its motors each time it moves.