Is there a CSPRNG capable of generating a large sequence of numbers from a single seed? In my CS course i am in dire need of a CSPRNG that can create N numbers starting from a single key.
Something like this:

CSPRNG(x):
Input 64 bit int;
Output Array of CSR 64 bit Ints;

Does something like this exists? Which ones?
Or is almost every CSPRNG like this?
A PRNG, by definition, outputs a sequence of arbitrary length from a finite state. So you can get as many chunks of 64 bits as you may want from any PRNG. Or, said otherwise: if you have something that gets stuck at some point, then it is not a PRNG.

Such a PRNG is said to be "cryptographically secure" (that's the "CS" in "CSPRNG") if the output bits are unpredictable; that is, given n successive output bits from the PRNG, it is computationally unfeasible to predict the next bit with probability substantially better than 0.5. If you use a 64-bit seed, then there are only 264 possible initial states for the PRNG, a large number, but not practically infinitely so. It is expensive but technologically possible to enumerate all possible 264 seed values, and get the corresponding PRNG output. In that case, it suffices to try all these seed values to pinpoint the one which produces the PRNG output observed so far; afterwards, the future output of the PRNG can be predicted with 100% accuracy, which means that the PRNG is not cryptographically secure.

If you want a really secure PRNG, then you need a larger seed space (typically 128 bits or more). Take care that a big seed is not sufficient to get a CSPRNG; it is merely a necessary condition, to prevent the exhaustive search attack described above.

For some examples of CSPRNG, see NIST SP 800-90A. That document actually separates the "PRNG" part (they call it "DRBG", the "D" meaning "deterministic") from the seeding, and enforce regular re-seeding. You can also get a CSPRNG from any good stream cipher (e.g. the ones from the eSTREAM portfolio) or by running a block cipher (with large blocks, 128 bits or more) in CTR mode.

There are a number of published PRNG (e.g. the Mersenne Twister) which are not meant for cryptography, and fail to be unpredictable by attackers. Such PRNG are good enough for non-adversarial situations (e.g. simulation of physics experiments) but not for anything related to security.