Just a 0.1uF decoupling capacitor should be fine (on each . If after installing the, you see erratic behavior, consider a larger capacitor. But, the 5V supply ("Mains Adaptor" etc.) should have its own adequate filtering (though, the cheaper ones do tend to be a bit "noisy", so more filtering might be needed.
Also, are those discrete LEDs or are they installed on a "module". If the latter, they probably already have decoupling caps. Otherwise, the datasheet, on it's "Typical Application Circuit" shows a 100nF cap on each WS2812B chip, and as raschemmel said, mounted as close to the VCC and Gnd pins as possible. Also, it should be a ceramic capacitor designed for decoupling.
Here's a link to the datasheet: http://www.sinistercircuits.com/docs/WS2812B_18031509254757.pdf
Also, consider that the current demand on the supply lines increases as more LEDs are added to the circuit (parallel currents add up), so be sure to use a conductor size that will supply enough current without a significant voltage drop. Also, the power supply (aka the "mains adaptor" etc) must be able to supply enough current to power that many LEDs. As Wawa pointed out, that could be as much as 2.2 Amps! Ask, if that doesn't make sense.
BTW: 100nF is the same thing as 0.1uF
Note to raschemmel: a capacitor value magnitude doesn't suggest the application. The application suggests the magnitude. A 1000uF capacitor may be used for filtering, but it may also be used for other things, too, like timing, or rapid discharge (e.g. in a xenon strobe light, or a pulse laser). And a 0.1uF cap has many uses, and there are many types.