The problem's that I have quite a lot of choices, hundreds, as a matter of fact. So how should I narrow down my list even further, given that I don't care about other stuff, such as campus size or location?

Consider how likely you are to get into each college, and then maximize the perceived quality of the institution, and the employment rate/salary and/or gradschool/medschool application success of its Biology grads.

(Incidentally, I semi-strongly recommend you consider the student body/social life as well.)

Moreover, to how many colleges should I apply? As far as I know, most people apply to 6-9 colleges, but some even apply to 20! I guess that by applying to as many colleges possible, my chances of admission go up. But, I probably won't have time to write hundreds of admission essays, or the money to send in my application to all these colleges.

There are two overlapping solutions. To reduce costs, you can apply early to one or two schools that are either your top schools (meaning that if you get in, you will not apply anywhere else) or ones that are high-safety/low-realistic, such that you are likely to get in, and you will only apply to reach schools during regular admission. One significant issue is that early decisions usually come out around December 15th, while most regular admissions are due in around January 1st; this means you will still need essays pre-written and edited, and for your school to send in your grades and recommendation letters ahead of time in case you do apply. (This method also gives you some info on which others schools you should apply to.)

(Though this can lead to lulz- I know a girl who applied early to Stanford, wrote up applications for literally 20 other schools, got in early, and never sent in the other applications.)

Also, you can try to see if you can get admission fee waivers, which eliminate admission fees for up to 4 colleges.

In addition, you may want to consider colleges with "rolling admissions". I have no experience with it; if you want more info, IIRC curiouskid has some experience.

For essay writing, it's often possible to reuse essays, with some editing, from school to school. (I had to do this extensively because I started writing my essays in June...using the previous year's prompts...)

Lastly, as my objective is to gain admission somewhere, should I only apply to colleges with acceptance rates above a certain percentage? What should that percentage be?

Just looking at percentages doesn't tell you enough about the school or you. eg some colleges had oddly high admission rates because only top students apply; Caltech would be vastly tougher to get into if you were very good in the humanities than if you were very good in STEM subjects. I don't know enough about biology programs to give you any specific examples, but this is a good thing to look into further.

How high you should aim in your college applications depends significantly on you- your grades, scores, extracurriculars, etc.