I have read that you should own at least the half of your tokens in a DAO to avoid attacks. Is this true? Does it applies for crowdsales too? If so, crowdsale should be just giving less than the half of their tokens? What kind of attacks are probable when this happens?
Just to add to the answerof cappy, it seems you are getting confused between an attack at the ethereum network level and an attack against your contract.
When we talk of 51pct attack, its generally at the network level, where there is a consensus mechanism. For this mechanism to work well, we rely on the assumption that no single entity own too big of a share of the hashing power. Any entity owning 51pct of the hashing power would definetly control the network, but some research show that under certain conditions an attacker would only need about 30pct of the hashing power.
As of attacks on a smart contract itself, it generally means someone who try to hack it by exploiting a flaw in the programming logic and calling the contract methods so as to take advantage of this flaw (i.e like in the dao). But in most cases, there is no consensus mechanism at the tokens level. So in most cases a 51pct attack is not relevant to the smart contract itself.