What is the max WS2812 pixels than can be used with one Uno?

What is the max WS2812 pixels than can be used with one Uno? I want to bild a 20x15 led matrix but I dont know the max LED pixels number. Or use mega instead?

It will handle 300 just fine.
Not power them, but provide the data. Any Arduino is equally capable of this, you don’t need a Mega.
You will need a separate power source.

I know already have a 5V 20A 100W power supply. A real question is how I can use these 600 leds whit the glediator software? Because the simple code for ws2812 can handle 512 led. I they don't do a 1024 version like for Mega whit ws2801 leds

You don't have to use only 1 data line. In fact you probably shouldn't so your refresh isn't horrible.

INTP:
You don't have to use only 1 data line. In fact you probably shouldn't so your refresh isn't horrible.

Ah, so does using more than one data line improve the refresh rate? Do the Adafruit or fastled libraries support sending data to different strips simultaneously, rather than one after the other? If not, it would take the same time to refresh all your leds. I would be quite surprised if a 16MHz Arduino could send multiple data streams simultaneously.

I suppose that, even if the Arduno can only send data to one strip at a time, you might be able to separate parts of your display such that some parts don't change very often, allowing other parts to be updated more frequently.

Voltage drop on data line is prob the bigger issue

I see a bigger display and they dot have any problem whit data line voltage drop. but I still don't know how I treat the 600 leds. I want to use the glediator software and a short script but these script cant't handle the 600 leds just 512...

Why would there be voltage drop on the data line? Arduino only sees one input pin, the data line is daisy chained from there.
600 LEDs = 1800 bytes of data to send out, whether 4 strings of 150 LED, 3 of 200, 2 of 300.
(Unless some strings get the exact same data, and can be wired in parallel).
I think the library updates at a 400 KHz rate? 1/400000/bit * 8 bits/byte = 20uS/byte * 1800 bytes = 36mS to update them all. That's a 27.8 Hz refresh rate, little faster than the 24 Hz movie theaters used to do.

I suppose if you did one chunk, then the next, then another, the first one to get its data could have its outputs updated while the 2nd gets its data, etc. Vs all being updated together after all the data is loaded. You'd have to determine if that kind of updating was visually acceptable.

Can't help you on the 512 vs 600 issue.

Because voltage always drops since nothing has 0 resistance?

INTP:
Because voltage always drops since nothing has 0 resistance?

The point is that each WS2812 regenerates the data signal, so it is only driving the data the half inch or so that the LEDs are apart. So there is no voltage drop to worry about. It is not cumulative.

Must've been thinking of the power rail, then