The sample rate depends on your specific needs. In the case of Rx, a lower sample rate will produce less data, which will require less storage, processing, etc. But it will also "see" less bandwidth. So, if you're looking for a signal that's 1 MHz wide, 2.5MS/s will be fine. But if you're looking for a signal that's 10 MHz wide, 2.5MS/s won't be enough. Since they're complex samples, 2.5MS/s will give you 2.5 MHz of bandwidth, though the outer edges will begin to roll off, so I'd recommend sampling at least a little bit higher than the bandwidth you need.
If you're not familiar with the theory, you should read up on sampling, Nyquist, etc.
For a practical discussion of sampling rates at the AD9361 receiver relating to symbol rates, signal bandwidth in the context of building a QPSK receiver targetting PicoZed SDR, see 'Software-Defined Radio with PicoZed SDR SpeedWay Workshop 2015' https://products.avnet.com/wps/portal/ema/event/software-defined-radio-picozed-sdr-speedway-workshop-2015/Lab 1 'Day 1 PM Lab: QPSK Receiver with Radio-in-the-Loop'
I have PicoZed™ SDR SOM Z7035/AD9361 Development Kit.
On the repository https://github.com/analogdevicesinc/libiio in examples I found ad9361-iiostream.c.
In this file author set sample rate 2.5 MS/s for rx and tx.
In datasheets I did not found information about recommended sample rate for AD9361.
Can somebody help me to know what is recommended sample rate for AD9361?