Hello. I was wondering if a 3m ws2812b led strip can be powered only by arduino or will I need an additional 5V power supply.
Wonder no more, there is no way on earth you can do without an external supply. And quite a big one at that. Allow 60mA for each LED you have.
This expression seems to come up here often - "powered by Arduino". It is a very strange concept, suggesting a very limited understanding of electronics.
An Arduino - such as a UNO, Nano, Pro Mini, Leonardo and similar - in no way resembles a power supply. A "power supply" is a component you plug into a "power point" or "utility outlet". Or devices may be powered by a battery of suitable capacity. But that and an Arduino are totally different things.
The confusion is usually encouraged by descriptions in the Arduino references that there is an on-board regulator on these boards. This is true - there is a regulator on the board and it can be used under limited circumstances to power the Arduino board and only the Arduino board. Essentially nothing else as the regulator, whatever its rated capacity, has no effective heatsink and will overheat and (hopefully safely) shut down if required to provide more current than the microcontroller itself and a few indicator LEDs at 10 or 20 mA each.
If you power the Arduino versions which have one, with 5 V via the USB jack, then up to the limit of the on-board protective fuse and power switching circuitry, you can pass the 5 V through to the "5V" terminal on the board and use it to power other devices - such as LEDs - up to the limit of that fuse, nominally 500 mA. But in general, the Arduino is simply not a "power supply" in any sense, so referring to "powering" something by it is meaningless.
As an aside, everything above applies to the thing described as a "MB102 power supply module" in its various versions. You can use it to supply 5 V to a few logic ICs or an ATMega328 chip and a few indicator LEDs at 10 or 20 mA each. That is its purpose. If you need to power something that does not fit on the breadboard or requires more than 100 mA, then you need a suitable power supply. If you can find the adapter, you can plug a 5 V power supply such as a "phone charger" into the USB port on the MB102 power supply module and have it reticulate that 5 V to the side rails on the motherboard up to 500 mA or a little more; this is its purpose - a convenient power adapter; not a power "supply".