haloalkane to amine

In my notes it says that we need excess ammonia at roon temperature but need to use a high pressure in order to minimise further subsitution

If we want to minimise further substitution won't EXCESS ammonia encourage further substitution though?

If you used excess ammonia on a haloalkane you will first form a primary amine. Now if you had excess haloalkane at this point then your primary amine will react with the haloalkane to form a secondary amine. The secondary amine will then react with the haloalkane to form a tertiary amine etc...

If you use excess ammonia you form a primary amine with all the haloalkanes until it is all used up

Does that make sense?