Typical number of samples per symbol
2 to 4 samples per symbol is common in my experience. The incentive for using fewer samples per symbol is an overall lower sample rate (and thus less processing load). It depends what point in the system you're talking about, but you also need to take the excess bandwidth of your pulse shape into account to ensure that the sample rate is high enough to avoid aliasing.

In some parts of the processing chain, you can get away with fewer samples per symbol than you might expect given the Nyquist rate and the signal's bandwidth. If the signal is perfectly synchronized, for example, then you don't need additional oversampling. In that case, you can use just one sample per symbol (basically, the symbol values themselves). You can then use that and the known pulse shape to reconstruct the signal to whatever fidelity you desire. 

In the absence of perfect synchronization (for example, in the various receiver blocks that perform frequency, phase, and/or timing synchronization), you'll have some amount of oversampling. In the literature you'll find 2 samples per symbol as a desirable target rate, because it's really the smallest that you're realistically going to achieve. You can always go higher if you need to. It just results in a higher sample rate (so more computational work)