There are only two answers to the question of how many spaces you should leave after a period. The answer is one space or two spaces. Three spaces would be ridiculous. MLA Style and APA Style have different recommendations but most style guides recommend using a single space.

MLA Style - One Space

The MLA Style Center provides the following answer: "Leave one space after a period or other concluding punctuation mark, unless your instructor prefers two spaces. Whichever spacing you choose, be sure to use it consistently throughout your paper."

Therefore, if you are following MLA style you should use one space unless your professor says otherwise.

Chicago Manual of Style - One Space

In a Q&A, the Chicago Manual of Style says, "CMOS follows the industry standard of one space after a period."

APA Style - Two Spaces

APA Style recommends using two spaces after a period or concluding punctuation mark.

An APA Style blog entry from 2009 says: "The new edition of the Publication Manual recommends that authors include two spaces after each period in draft manuscripts. For many readers, especially those tasked with reading stacks of term papers or reviewing manuscripts submitted for publication, this new recommendation will help ease their reading by breaking up the text into manageable, more easily recognizable chunks."

The blog entry also notes that the "usual convention for published works remains one space after each period."

The Proofreading Pulse notes that the APA is the one major holdout among stylebooks recommending two spaces.

Scientific Study

There is widespread disagreement as to whether two spaces are better than one space. Science recently tried to solve the problem. This 2018 study from researchers at Skidmore College provided encouragement to two-spacers (people who use - or are in favor of - using two spaces after a sentence).

The Atlantic reports that the recent study found using two spaces after periods helped increase reading speeds slightly.

The Altantic notes that the sample size used in the study was unfortunately on the small side. It involved just sixty college students.