Ted Cruz isn't dumb, he had Vincent Harris before, and no doubt has learned many a lesson in the importance of database-building and digital game. He tweets himself, and is talking directly with supporters all the time. There also probably have excellent people running his digital operation. Not as fancy as Rand's, probably the second best in the field. I could easily see him trying to run a Ron Paul '12-style campaign based on engaging core grassroots conservatives, directing their activism towards winning caucuses and conventions - not to necessarily win the nomination, but to build a large loyal following like the Liberty Movement, so that he will be the top candidate with a strong ground game from the get-go for the next election.
18 000 is 14,8% of the 2012 total caucus vote of 121 501 voters. Very decent, and I don't doubt that they like Cruz a lot, but there is no guarantee that they will actually turn out to vote, or that they won't change their mind before the actual caucuses.
I'd have to assume, with all the talk of Rand's digital ground game, that they are building massive lists in the key states as well, and actively engaging with them. He should already have massive lists on beforehand from the 2012-campaign, The Campaign for Liberty, YAL, the Saber-people and continuous list-building as a Senator for 4,5 years. There are 600 000 registered republicans, and many independents that can also participate in Iowa. If you can convince 30 - 40 000 of them to show up and vote for you in the caucus, you will probably win with this many candidates running. (I suppose total votes will be higher in 2016 than 2012 because of more, and better candidates) In other words, you need far less than 5% of eligible Republican voters and Republican-leaning voters to show up for you to win. Should not be impossible to pull off for Rand at all. There are probably enough libertarians alone to pull it off. But, by the same logic, Cruz can pull it off as well with a good enough campaign. He has enough cash to compete in a big way. But Rand should have a head start, and has a lot more going on for him, especially if he get's to push his electability argument in a big way during the campaign.
The interesting thing is,

here is a very conservative candidate, (ok, with some twists, but most conservatives can manage - though some will dissent on foreign policy) that is polling better than any moderate or establishment candidate - and being essentially in a tie - and gaining - against with the most formidable democratic candidate in recent memory. If Rand can win Iowa in addition, then he will have changed the rules of the game completely. The conservative base easily got the numbers to elect whoever they want to the nomination. They have just never elected one since Goldwater since they all would suck and loose in the general election, with independents not favoring these traditional conservative candidates. (Together with donor influence, of course) With a widespread belief in the conservative base that they would actually stand a good chance in the general election with Rand, they will be forced to consider abandoning the likes of Cruz, Rubio and Walker in favor of Rand. Some will naturally defect to Jeb Bush, as Rubio, for instance, when he eventually backs out, probably will endorse Bush. But then, with a larger, and much more engaged base than the establishment, you can defeat their huge money, and win the nomination. Hopefully!