Just continuing the conversation that I'm late to, technically speaking, you do not use actual numerals until you get to ten, and you do not start sentences with numerals either.
Right (or at least, more formal):
Ten people criticized Gary for his use of parenthesis.
I have done it myself three times that I can remember.
There are 26 different rules for writing numbers in a sentence.
Wrong (or at least, frowned on):
10 people criticized Gary for his use of parenthesis.
I have done it myself 3 times that I can remember.
For me, someone that writes a fair amount of formal letters, I generally write numbers below ten as "six (6)" or "eight (8)". I either use "25" or "twenty-five" depending on the context; in a main paragraph or in a reference to days (as in "we will be paid in .... days"), I use "twenty-five (25)", in a bullet, a prices, or dates, "25".
Oh, and I like commas. The more the merrier.