The influence of fluctuations of pump beam parameters on the coherent combining efficiency of femtosecond pulses amplified in parametric amplifies based on LBO crystals under multiple-beam pumping is analysed. It is shown that in transition from a single-beam to two-beam pumping, the sensitivity of the parameters of amplified radiation to angular instability of the pump beams sharply increases. With increasing number of the pump beams, the sensitivity of the parameters to fluctuations of the pump beam propagation angle weakly varies. It is found that if a substantial number (n >> 1) of 2-cm-diameter beams amplified in parametric amplifiers under multiple-beam pumping are coherently combined, then obtaining the 98 % efficiency of this process requires the angular stabilisation of the pump beams better than 50 mrad. Fluctuations of other pump beam parameters such as energy, phase aberrations, and relative jitter either affect only the amplitude of the pulse amplified or the influence on the phase profile of the amplified beam is at most 10-2 rad, which should not noticeably affect the coherent combining of amplified pulses.