What alphanumeric string length can be used to guarantee no hash collisions from CRC-64? If I'm hashing alphanumeric strings (chars in the set 0-9, a-z, and A-Z) with a CRC-64 hashing function, how long can my strings be while guaranteeing no hash collisions?

Stated differently: If I have a set of all CRC-64 checksums computed from alphanumeric strings  $0$ to $N$ bytes, how large can $N$ be before I see a collision in the set?
If you have 62 chars you can transform 62 letters ($10+26+26$) in 6 bit number (approx). CRC is guaranteed to be unique mapping (Injective function) as long as input is shorter than output  – you can have at most 10 letters, but not 11 since $62^{10} &lt; 2^{64} &lt; 62^{11}$.

It same goes whit most other hash functions. Lets say that you have hash function that takes only 64 bits and makes 64 bit output and is bijective (like a symmetric cypher). That mean you will not have collision as long as your input is shorter than 64 bits.

If you have mappings from all 64 bit inputs you are 100% sure that each new input will produce collision (because there is no value that is unused)