Number+Rules

From the MLA (Modern Language Association) Handbook:

1.) If a number can be spelled out in one or two words, it MUST be spelled out. EX - one thousand; forty-seven; eighty; seventeen trillion; etc.

2.) If the number is at the beginning of a sentence, it must be spelled out no matter how long it is. (A good solution is to rearrange the sentence so the number is not at the beginning)

EX - 147,886 people attended Friday's concert. (No good!) On Friday, 147,886 people attended the concert. (Better!)