Federal laws are so numerous and complicated that even the federal government cannot NUMBER them. they literally can't even count them all. You and everyone else in this country is guilty of many of them.
but hey, if they decide to go after you for some reason, and bring you up on federal charges, please let me know how it goes in court accusing the judge and the prosecutor of "must have done something wrong too"
This is just a meaningless thing to say. The reason for the difficulty in answering is because the question ("how many laws are there") is flawed because it implies a static number. Some rules are temporary (e.g.: a safety zone around a small area for X hours while underwater explosives are used for dredging from June Y-Z). They receive temporary CFR citations that start with T and they expire when they are no longer in effect.
The number changes, it's not "uncountable" -- this meme needs to die.
Oh by the way, these are also by and large submitted to the Federal Register for people to have a chance to read and submit comments and request public meetings.
How realistic is it for a normal person, with a full-time job, perhaps a family, to keep track of every relevant Federal Register submission which may affect them?
Should they be keeping track of all newly-passed (and rescinded) laws, as well?
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On a separate note, I wonder what the result would be of a legal system in which all laws have a mandatory expiry, and must be re-ratified/voted on to remain applicable. The period would have to be long, but not too long (somewhere 10-20 years maybe?) so as not to backlog, but even that might be a benefit in keeping the number of laws from growing too fast.
The biggest problems I can imagine is prosecution of subsequently expired laws, and how legal precedent could work, when it might have been made based on a wording different to the current.
Largely, no, because it would be a spectacular waste of their time. Why? Because the regulations are usually microscopically focused on class of actors that (a) know they exist, and (b) have a vested interest in staying current.
Moxie Marlinspike's example of the undersized lobster rule is especially telling in how it goes against the very point he was attempting to make. On the enforcement side, this particular regulation likely affects fewer than 10,000 people in the entire country -- that is, people fishing commercially for lobsters. The public interest it serves is that fisheries are subject to the tragedy of the commons[1], and must be regulated to preserve these public goods. It's basically the equivalent of that little tag on your mattress that says "ILLEGAL TO RESELL" -- people joke about getting arrested for this. No, it's targeted at the vanishingly small number of people that sell mattresses commercially.
[1] http://en.wikipedia.org/wiki/Tragedy_of_the_commons
EDIT: Also, just a note -- laws are passed by Congress. Regulations are promulgated by agencies that have been delegated some task or authority from Congress ("we don't have time to draft/debate/vote on every little piece of trivia regarding airplane part advisories or what plants are on the invasive species list or what next year's technical specifications for automobile safety are -- FAA, USDA, NTSB, you guys have the experts and scientists, so here's your authority, now go and figure out the details and make it happen")
Of course, any reasonable court would/should dismiss this, even in the unlikely instance that someone would decide to prosecute, but the problem is that is a dangerous thing to rely on. And if it were recorded by not prosecuted, it's now a liability that can be used against you for other reasons, until the statute of limitations applies.
If the law prohibited selling, or possession in a commercial context (large lobstering boat, etc), it would be reasonable. The damage to lobster stocks is likely[1] due to the large commercial concerns, rather than individuals catching small numbers for personal consumption.
Likewise for the 'it's targeted at the vanishingly small number [...]'. If the law isn't specifically targeted at them, why not? One good reason would be the creation of loopholes that allow it to be broken without penalty, but if the alternative is allowing perfectly normal activities to bear the possibility of prosecution, subject to the whims of the legal system, I think I'd be in favour of being precise.
[1] wild-ass guess here, but I think reasonable