Some monsters are capable of multiple drops if they are rolled (like a Kobold has a chance for a drop, and then has a chance for dropping a couple of things). Some have different types of drops that can appear, others have one
(the Air elementals you and Saito are fighting actually have two items they can drop, but the second one is extremely rare)
Also yes there will be titles, but no one has gotten any yet (well... actually Ren should have gotten a few Imp Slaying titles by now for killing all of the imps he has killed and one zombie slaying title since he did grind zombies for a bit and kill a good small bunch of them, and he has killed alot during his month of having the power. Also titles for the two boss monsters he has killed). Yea, I completely forgot about titles. XD
And maybe the others should have gotten a title for all the Imps they have slain... well for continuity's sake we will say that they haven't killed enough yet (maybe one more grinding of Imps would get them the title).
As for boss mobs they have a chance of spawning based upon how many of the regular mob you have killed, so it is random. But yea, grind enough regular mobs and a boss mob will eventually have such a high chance of appearing that it will pretty much happen unless the roll is really, really bad/good (I say bad/good cause it depends upon whether or not you want to fight boss mobs, cause if you're in bad shape then fighting one is not the best idea).
So yea, even if you do grind alot a boss monster might not appear, but the number stacks. So you could grind 20 or even 50 Imps on one day, have no boss monster appear, then grind like 5 or 10 more the next day and have one appear since the chance from the 20 or 50 you grind carries over until one appears).