The main limitation on use of NeoPixels on many microcontrollers is memory. The microcontroller, in general, has to reserve at least three bytes for the red, green, and blue pixel values (four for RGBW pixels which adds a white component). The more pixels to control, the more memory is used.
M0 based products like Circuit Playground Express and Feather M0 Express can usually drive about 100 pixels easily. With the Feather M4 Express, this is increased. The maximum will have to be something you test as with code, the amount of free RAM varies. 300 RGB pixels may be possible on Feather M4.
If you plan to drive a large number of pixels, like for a whole room or art project, look at the full range Adafruit Products and guides. And catch us on the Adafruit forums under Glowy Things and on the Adafruit Discord channel. Folks can guide you to something if it is larger than can be accommodated with this introductory guide.
Crickit is designed to handle a decent amount of power (current) to your NeoPixel projects. Power = current x voltage, so with a constant 5 volts, power increases proportionally to current needs.
You will need to get an appropriate power supply to handle the peak (maximum) current you plan to draw in your project.
For a static (non-mobile) project, Adafruit recommends the 5V 2 Amp supply for Crickit use.
If you plan on lots of motors and lots of NeoPixels, you can use a power supply up to 4 amps. If you were to use 100 LEDs on full white, you might draw 4 amps according to Ladyada. If you do not run the LEDs all at full brightness and color, your project will use less current and save your eyeballs from the bright light.
If you don't run your NeoPixels full bright, say 30%, you may get up to 300 pixels running if you have enough memory free on the microcontroller you are using on your Crickit.
If you are running into limitations on the Crickit current use, you may have to look at a more sophisticated design. More often than not you'll run out of memory sooner than current capacity for the LEDs.
A lot of issues that users see if their project are not working is that they have RGBW NeoPixels driven by RGB code. You get a lot of perceived weirdness that strips light up part way, colors are random, some light up in odd colors, etc.
Check your NeoPixels to see if they are RGB or RGBW and use the correct code accordingly. And do not mix RGB and RGBW NeoPixels in the same circuit as the software is usually designed for one or another.
The NeoPixel Uberguide may help.