Are you planning on going outside with it? If so, the answer is "Probably less than 5mW" - the legal limit for a Class 3R laser. Anything higher presents a risk of blinding.
So, 10,000x less power than the lasers on that webpage above.
Those seem to show up in the 500mW+ range, with 2000mW being enough for most softwoods (among other things). Industrial applications seem to range from 50 to 5000+ watts, but the lasers used there are likely to be better focused than the laser pointers you'd find off a site like the above.
And if you buy a laser CNC machine, it'll typically have interlocks present to keep you from accidentally blinding yourself. (which is a real possibility - if the material you're cutting has shiny inclusions, the wrong reflection can blind you)
"The wrong reflection" doesn't give the right impression. The point a 100W laser projects against e.g. a wall, when viewed at 1m distance, can damage your eye. Even if perfectly diffuse. Even if invisible.
I recommend taking a laser safety training class. At least the ones focusing mostly on show lasers are widely available to the general public, and not expensive. It'll cover the optical dangers at least. Possibly it'll also build some respect for lasers, and maybe make certain things legal for you (important esp. after something goes wrong).
For machining lasers, there's a bunch of extra dangers to you _and_ the machine (e.g. particle exhaust, fumes, certain materials, fire risk (and secondary risk when putting it out!), very high voltages, human errors (I'm serious! and incl. both you and other/untrained people), legal requirements, overheating, electronical failures of the controller or interlock or (very relevant for hobbyists) plain bugs).
On the upside, hobby CNC lasers are often in the order of 100W (appropriate for cutting plastic/wood, engraving glass, and marking metal (with marking spray)), and CO2-based (10μm). A small stone brick will easily work as a beamdump. Basically _any_ thin plastic sheet will easily block secondary beams at this wavelength (so counter-intuitively the laser being invisible is an advantage!). And if the laser is dispersed enough possibly even direct hits, for a very short amount of time (up to seconds). (This is no excuse to save on the goggles.)
Do not trust cheap "complete, in a safe box" CNC lasers bought on the internet. They work, but require _extensive_ rebuilding to be safe.
It's possible and fun to work with lasers as a hobby. But please stay safe! If you can, don't go alone, find a fablab or hackerspace, maybe even build it there.
How thin of softwoods are you cutting? Event consumer-grade products like Glowforge are 40W, so I have a hard time believing a 5W laser is practical for much. (FWIW I maintained a 65W CO2 laser cutter for a while.)
So, 10,000x less power than the lasers on that webpage above.