Question

what do we get by adding excess ammonia to 2-cloro-2-methylpropane?

Answer 1

RCl + NH3-> RNH2+HCl. Further reactions are possible but since NH3 is in excess reaction should stop here.

Answer 2

is there a possibility of elimination instead of substituion? how to decide which one will occur
?

Answer 3

In this case elimination is not possible,
NH3 + RCl-> RNH2Cl. Elimination is ruled out for this.
In general both can occur but elimination is preferred over substitution at higher temperatures.

Answer 4

and what would happen if nh3 was taken in small quantity?

Answer 5

Then the reaction would continue.
RNH2+R-Cl->R2NH +HCl
R2NH+RCl-> R3N + HCl
R3N+R-Cl-> R4N+ Cl-.
The reaction can proceed till any of these products So this reaction is generally of no practical application due to low yield.