Why do we need to add Al in excess for the reaction between a nitrate and a base to give ammonia?
Hydroxide and nitrate alone do not react  with each other.

Their reaction with aluminium is the part of the classical quantitative determination of nitrate content by distillation of produced ammonia and by the back acid titration.  

Nitrite + ammonium ions interfere, but the former can be eliminated due their high reactivity, the latter can be determined and/or eliminated by prior distillation without aluminium. 

Aluminium acts as a reduction agents, reducing nitrates to ammonia, forming alkali aluminates.

$$\ce{3 NO3-(aq) + 8 Al(s) + 5 OH-(aq) + 18 H2O(l) -&gt; 3 NH3(g) + 8 [Al(OH)4]-(aq)}$$

Generally, aluminium need not to be in an excess. But if there is the desire the all nitrate ions react quantitatively, aluminium and hydroxide must be in excess.

Usually, the Devarda's alloy is used, instead of pure aluminium, containing also copper and zinc.