When writing a essay. How many spaces after a .

after ,

"Is this correct".

Standard form calls for two spaces after a period and one space after a comma.

The question, "Is this correct?" needs a question mark at the end -- followed by two spaces.

When writing an essay I would say two spaces after the period and one space after the comma.

"Is this correct".
No that is not correct. Usually the period is inside the quotations. The only exception is when the ending unit is either a single letter or a number, then the period would be outside the quotes. example:

My favorite number is "8".
My favorite letter is "D".

-Don

When writing an essay, it is important to follow the appropriate formatting rules. According to most style guides, including the Modern Language Association (MLA) and the American Psychological Association (APA), you should use only one space after a period. This means that after both a period and a comma, you should use a single space.

So, in the example you provided, "Is this correct", you should use one space after the period.