Why is Spaceballs specifically numbered Chapter XI?

Good answer by @sanpaco and it may be partially true, but according to Mel Brooks himself, there is a different reason.

I was on holiday in Australia in 1997, when I happened to watch part of a live television-show broadcast from a SF fair/conference event. (Now that I think of it, it may have been a recording of a live show that took place earlier.) Mel Brooks was there and he answered questions from the audience.

I will try to re-produce the gist of the exchange as I remember it:

Q: Why is the Spaceballs movie specifically called number 11? That it should have some number is obvious giving that is a Star Wars parody, but why 11?
A: You know, we could have picked any random number, or called it 4, just like the original Star Wars movie, but that would have been too obvious.
You ever seen the movie This is Spinal tap? They took it up to 11 and that is just what we did too.

Taking something "up to eleven" has become a general idiom for something that is done "over the top". Of course, Mel could have made it up on the spot, but it sort of makes sense for Spaceballs.

EDIT: To all the people that up-voted this: My thanks! Unfortunately I have not been able to find a proper source for this. I tried to do some digging, but didn't get anywhere. It was 20 years ago and I can't find any info online what was shown on Australian TV channels back in the day. The fact I turned on the TV and dropped in to the program halfway didn't help either. I'm not even sure the details of which SF event/show it was all about (that would have given another search key) were even mentioned at all in the part that I saw. So for now I have to leave it at this point.

Theory #1: It is a joke about chapter eleven bankruptcy

The only thing I can think of that is specifically related to the number 11 is possibly some kind of obscure joke about Chapter Eleven Bankruptcy. This might be a bit of a stretch, but it wouldn't be the first time a joke in a Mel Brook's movie seemed a little vague.

Some evidence supporting this idea:

• The theme of being short of money is prevalent in the movie. Lone Star owes money to Pizza the Hut for example
• Spaceballs planet is running out of air (bankrupt of air?)
• The working title of the movie is shown in the scene with the instant video cassette - "Chapter Eleven: The Spaceballs Grow Desperate"
• At the end we are invited to watch the sequel "Spaceballs 2: The Search for More Money"

It also seems that others have made the same connection to this being a joke about bankruptcy:

Opening on "Chapter Eleven" (a nice joke about bankruptcy), a familiar title crawl informs us that Planet Spaceball, led by President Skroob (Mel Brooks) has used up all of its air and plans to steal the air from the Planet Druidia, led by King Roland (Dick Van Patten).

Theory #2: It is a joke about the seeming randomness of Empire Strikes Back including "Episode V" in the crawler

Another theory is that it is a joke playing on the confusion of audiences when The Empire Strikes Back opening crawl included the title "Episode V - The Empire Strikes Back" while the original release of Episode IV did not include the episode number and was simply known as "Star Wars".

IMDb's FAQ for Star Wars: Episode IV - A New Hope gives some insight on the history of this and the confusion of audiences:

When the film was originally released in 1977, it was simply referred to as "Star Wars"; though supposedly, George Lucas had intended to include "Episode IV" and "A New Hope" in the opening crawl, but Twentieth Century Fox did not want Lucas to do so because they thought it would confuse audiences, since there were never any other episodes released before it. After the commercial success of the original Star Wars, Lucas was able to continue with the multi-film epic he originally envisioned. The first sequel, The Empire Strikes Back, was released in 1980 and bore the full title of "Star Wars, Episode V, The Empire Strikes Back" in the title crawl, although it was referred to only as "The Empire Strikes Back" as the title of its commercial release. It was the "Episode V" appearing in the opening crawl which originally confused those members of the audience who had not been made aware of what Lucas was explaining, that the original Star Wars was now to be understood to be the fourth part of a nine-part series. The original Star Wars was re-released in 1981 with a new title "Star Wars, Episode IV, A New Hope" in the title crawl. This title appeared on all subsequent re-releases and versions from then on (though the original version was released on DVD in 2006, which shows the title crawl in its original form). All subsequent Star Wars films have followed this new naming structure, although "Star Wars" often refers specifically to the 1977 film.