Should I capitalise the first letter when a sentence starts with a number?

Since you ask about sentences that begin with a number, it seems relevant to note that many style guides advise against using a numeral at the beginning of a sentence. And if you spell out the opening number as a word, the question of whether the next word in the sentence should be capitalized doesn't come up. Here are some stylebook guidelines on this question.

From The New York Times Manual of Stye and Usage (1999):

In ordinary news copy, spell any number that begins a sentence: Five hundred delegates attended.

From The Associated Press Stylebook and Briefing on Media Law (2002):

SENTENCE START: Spell out a numeral at the beginning of a sentence. If necessary, recast the sentence. There is one exception—a numeral that identifies a calendar year.

Wrong: 993 freshmen entered the college last year.

Right: Last year 993 freshmen entered the college.

Right: 1976 was a very good year.

From The Chicago Manual of Style, fifteenth edition (2003):

9.5 Number beginning a sentence. When a number begins a sentence, it is always spelled out. To avoid awkwardness, a sentence should be recast.

[Examples:] One hundred and ten candidates were accepted. (And may be omitted.)

or

In all, 110 candidates were accepted.

From Words Into Type, third edition (1974):

Beginning a sentence. A numeral should not stand at the beginning of a sentence. A number so placed should be spelled out or the sentence reworded to bring the number elsewhere in the sentence.

Wrong: $36,000 was the amount of the debt.

Right: The amount of the debt was $36,000.

Wrong: 1941 saw our entry into the war.

Right: The year 1941 saw our entry into the war.

From The Oxford Guide to Style (2002):

Use words for all numbers at the beginning of sentences; to avoid spelling out cumbersome numbers, recast the sentence or use periphrasis, for example The year 1849 instead of Eighteen forty-nine.

The way you handle numbers at the beginning of sentences remains a style decision, but the style guides I consulted showed a remarkable degree of unanimity in favoring spelling out the number in that situation. The only dissenter on this point is the AP Stylebook, which recommends spelling years with numerals even at the beginning of a sentence. It bears observing that in its example of a sentence starting with a year—"1976 was a very good year"—AP rendered the second word, was, in all-lowercase letters.

Whether this advice applies to sentence fragments such as "96% real meat" is for you or your publisher to decide. If I were posting a fragment such as that one on, say, a butcher shop price card, I would feel free to please myself on number-versus-word, capitalization, and punctuation.