Generally the bulk of receivers and servos out there are happy with either 4.8V or 6V batteries. For most trainer and sport airplanes though a 4.8V, 1100mAh pack will be just fine. Depending on the airplane 5 flights should be no problem and possibly more (always check batteries throughout the flying day) The only exception might be if you are using a Spektrum/JR radio since they have had low-voltage reset issues in the past. Even though those problems are supposedly fixed, I'd still use a 6V, high capacity battery to be safe.

6V is also advantageous in some situations where a little more servo torque and speed are desired such as 3D aerobatics. Otherwise there is no need for anything more than 4.8V, definitely not for someone just starting out.