I am running a single player world, but I never see any doctor who mobs.  Just vanilla.  The config files don't seem to have any settings related to mobs, and I can't find anywhere online describing the config files either.

Are you possibly on peaceful mode? 

Nope, I get plenty of vanilla mobs

Some mobs spawn in special biomes like daleks they spawn in plane biome 

After about 8 hours I ran across a dalek.  since then (about another 8 hours) i've come across about 6 more.  Is there a way (in 1.16.5) to configure the mob spawn rate higher?