How do you know when to use quizá or quizás? Does it have to do with plurals or the sentence it is being used in'
quizás or quizá
They are exactly the same in every respect.
quizá = quizás
They have been in the dictionary since the 18th century, but at that time "quizá" was considered to be its original form. They have both been used indifferently both formally and informally for the last two centuries, though.
They are adverbs, and as such, they don't agree in gender, number or person with anything. If used at the beginning of a sentence, both indicative and subjunctive are used (although the latter is preferred) depending on the degree of confidence. If they are used after the verb, indicative must be used.
I needed this answer today, and here it was. Thanks, Lazarus!