What i know about the audio interface and the mixer is that they both turn analog signal into digital signal. But why should one buy an audio interface if they could buy a mixer its cheaper and has more slots to plug into. is there a ability on the audio interface that i'm missing?
What i know about the audio interface and the mixer is that they both turn analog signal into digital signal.
That's not right. Not all mixers have the ability to turn analog signals into digital signals.
Some mixers are all analog (no digital capability); some mixers are all digital (no analog capability); some mixers might have the ability to convert to digital.
Very cheap mixers often have no digital capability at all, or they might have the ability to convert just their main bus to digital (2 channels) as part of a USB interface.
But why should one buy an audio interface if they could buy a mixer its cheaper and has more slots to plug into. is there a ability on the audio interface that i'm missing?
If you want to get separate digital tracks into your computer to mix in your computer, the fact that the mixer has more slots to plug in to doesn't necessarily help you, because the mixer can't necessarily convert all those inputs to digital. It might have no digital capability at all, or it might only have (for example) 2-channel USB capability.
As an example, here's the Behringer X1204USB:
It has 12 inputs, but only a stereo USB interface. So it can only get 2 separate digital channels into your computer.
On the other hand, here's the Behringer UMC1820, an interface that costs a little more than that mixer:
This is an 18x20 interface, which means it has 18 separate digital inputs and 20 digital outputs.
If you want to record multiple tracks simultaneously and then mix them together in your computer, then it's much better to have many separate digital inputs. That's one reason why an audio interface might be better if you are using a computer.
You can buy mixers that can send all their inputs in to the computer via USB, such as this Soundcraft Signature 22 MTK:
...but that costs a lot more than the Behringer interface.
Of course if you only need to record 2 digital tracks at a time but you want multiple analogue inputs, an analogue mixer with stereo USB might be great for you. But in most markets they aren't usually cheaper than the cheapest stereo-only audio interfaces.