swadhin but rather ultrasonic signals of higher frequency range.
To me ultrasonic means >20 kHz so a sampling rate >44.1 kHz is needed to go very far into the ultrasonic range. Is that what you mean? The types of microphones you are considering won't give you very strong signal above 20 kHz, so the 44.1k samplerate is an acceptable match for those mics.
If what you mean is you want to record audio up to 20 kHz, then you do need 44.1 kHz sampling frequency, which means you have to dig into Bela core to try to work that out. Even so the result won't be high-fidelity.
swadhin If I use two bella boards, how do I synchronize them ?
This depends how closely you need the wav files synchronized. It may be they are close enough that you don't need to.
One way to synchronize them more tightly is to generate a digital output from one board at, say 1 pulse per second, and then record it on the digital input of the other boards since digital I/O is synchronized to the audio rate. You can use the period of the sync signal to compute the sampling frequency drift over the course of the recording and then resample the wave files from one source by the difference between the 2. I would assume the system clocks are pretty stable over the course of several seconds so I wouldn't expect you would need a sync pulse any faster than 1 second. Synchronization would be a post-processing task.
Synchronizing record stop/start times will be within 1 sample using a digital I/O input on each Bela driven from the same switch, so you would have a signal to ensure the files are time aligned at time-zero.
You could also synchronize in realtime by implementing a DPLL (digital PLL) based on the suggested sync clock and resampling as you record. Post-processing is easiest unless you intend to be doing some kind of realtime control you aren't mentioning. Probably 1 second pulses would be fine for this as long as you allow several seconds after startup for the control loop to lock and stabilize.
You're leaving a lot of mystery as to what you're trying to do. If you want high quality audio up to 20 kHz, you can't do it with 8 channels on Bela.
If all you are doing is recording 8 channels of wav files, then you might consider a >8-channel USB audio interface and skip Bela altogether unless you have some kind of real-time processing or control to do. This is reportedly supported by Linux out of the box:
http://www.presonus.com/products/AudioBox-1818VSL
96 kHz at 24 bits
Based on the types of things you are asking I'm getting the impression that you want high quality audio recordings, but the SAR converter inputs on Bela are not the correct type of interface for studio-quality recordings. You need to use the audio CODEC to get audio quality with low enough noise for anything above 10 kHz to be meaningful in a hi-fi sense. For 8 channels that means you need 4 Belas -- and that sounds expensive to me.