I have an idea to use WS2812B as animating light on some old equipment. And I have some doubt. The equipment is 12v so I want to use WS2812B directly on 12V, namely I want to use 5 WS2812b in pararalel, as they usually are on strips, BUT what resistor to use to limit current?
WS2812B are strictly 5V devices, so you'll need a regulator of some sort, not just a resistor. Either the classic LM7805 linear regulator (which will get hot) or an equivalent switching regulator.
Are you wanting to use 5 strips of WS2812B or are you talking about 5 individual WS2812B? Whichever it is, the data sheet says 3.5 - 5.3V for the power supply. Since the current drawn by each individual device will vary from 0 to about 50mA depending on what colour and intensity you have selected, a simple resistor isn't good enough. You will have to generate 5V from the 12V supply.
If it is only 5 individual WS2812B you may get away with a resistor and a 5V zener diode. If it is 5 strips you will need a 12V to 5V buck regulator of sufficient capacity to drive them all. A strip of 30 LEDs will draw up to 1.5A so 5 of them requires up to 7.5A. Alternatively a separate, cheap 5V 10A PSU, widely available on line.