What's the numbering system for spaghetti and does it matter?

Ok, here is the straight dope, directly from De Cecco customer service:

With reference to your question we would like to inform you that the numbers you mention do not have a logical criteria but are just code numbers which we give our products each time we make a new shape.

As an Italian, I really don't know the logic behind the number. I also asked my mother, to no avail. We don't know, but as far as my feeling goes, it's just a numbering system for the product, e.g. there's no implicit meaning into it.

As far as we worry about it or not, sure we do. Matching the wrong pasta with a given sauce is almost blasphemy and as a feedback for the mistake you can be frowned upon up to being openly insulted. I'm not kidding. For example, meaty stuff (such as ragu', also known as bolognese, but only outside of Italy) are matched with penne, tagliatelle, fusilli, and in general all egg-based pasta (e.g. strozzapreti, paglierini, pappardelle, spaghetti alla chitarra). Spaghetti bolognese is heresy for us.

Spicy sauces, such as puttanesca, amatriciana, carbonara and so on, require normal pasta, and can be matched with spaghetti, or also penne. Pesto always goes with bavette, although sometimes we use spaghetti for convenience, but I feel it unnatural.

To sum up, there is no simple rule, although instinctively, if you tell me a sauce (even invented brand new) I can tell you which pasta is appropriate and which one is not.

They are just a "product number", and it may vary for the same kind of pasta from a manufacturer to another.

It surely defines the tickness of the Pasta; tipically, more thicknesses corresponds to a greater number value.
Example for Barilla:

1. Capellini #1
2. Spaghettini #3
3. Spaghetti #5
4. Vermicellini #7
5. Vermicelli #8
6. Bucatini #9
7. Bavettine #12
8. Bavette #13

In Italy, they have a numbering system with corresponding names for each number. Larger numbers indicate thicker noodles. Some US manufacturers use the same numbering system. Here is a listing from an extrusion die manufacturer. See the full list here Pasta Shapes.pdf

#1  0.6 mm. 
#2  0.7 mm.
#3  0.8 mm.
#4  0.9 mm.
#5  1.1 mm.
#6  1.3 mm.
#7  1.5 mm.
#8  1.7 mm.
#9  1.9 mm.
#10 2.1 mm.
#11 2.3 mm.
#12 2.5 mm.

Found a link here http://www.sicilianculture.com/food/pasta.htm

"Numbered" Pasta Often you will see pasta with numbers on the package like Thin Spaghetti #9. Why? What does it mean? Well, in the "old days" there were waves of immigrants that came in to work in the factories. There were the Irish, the Asians, the Germans, the Italians and numerous other ethinc groups. Other than the Italians, none of these other groups really spoke the language, and were much less able to pronnounce or decern or know the difference between "spaghetti or spaghettini". So, before the days of automated computers, the factory managers had to get everyone straight, so it was much easier to say "today, we are making #9".

As far as I recall, I have seen numbers only when referring to spaghetti. Spaghetti #5 is the normal size, and spaghetti #8 (spaghettoni) are thicker; there are also spaghetti #3 (which in Italy are called spaghettini).

Normal spaghetti are always #5, but the thickness depends from the brand, in the same way shirt sizes depend from the brand.

Yes, it does refer to the size (in terms of thickness) of the pasta. I guess the number indicates the specific "drawing" (trafila in italian), where the higher is the number, the wider is the hole, and the thicker is the pasta. I don't think there is a standardized size for each hole so, depending on which trafila pasta producers use, the result might be different.

EDIT: apparently, the answer below is a 'common misconception'. See the other answers to this question. Learn something new every day!

Yes, there is a spaghetti numbering system. The smaller the number, the smaller the radius of the spaghetti.

For puttanesca, I've seen a suggestion of using #12.

Whether italians worry about this kinda thing - I dunno, but I imagine it depends on the Italian. I haven't run across recipes specifying size yet, and I don't look at it in the store (not even when i was in Italy).

I also have a great illustration for you about number spaghetti. :)