What is the IUPAC nomenclature of alkane with large numbers of C atoms (more than 200)?
The definitive answer is, of course, in the rules established by the IUPAC’s “Commission on nomenclature in organic chemistry”. The reference you are looking for is:


  “Extension of Rules A-1.1 and A-2.5 Concerning Numerical Terms Used in Organic Chemical Nomenclature”, Pure Appl. Chem., 1986, 58, 1693-1696


which can be found here as a PDF and here as an HTML version. In particular:


  NT-1.1 - The fundamental numerical terms for use in hydrocarbon names or as multiplying prefixes for simple features are given in the following list:
  
  \begin{array}{|c:c|c:c|c:c|c:c|}\hline
    1&amp;\rm \text{mono- or hen-} &amp;10 &amp;\rm \text{deca-}&amp;100&amp;\rm \text{hecta-}&amp; 1000&amp;\rm \text{kilia-} \\\hline
    2&amp;\rm \text{di- or do-}&amp;20&amp;\rm \text{icosa-}&amp;200&amp;\rm \text{dicta-}&amp;2000&amp;\rm \text{dilia-}\\\hline
    3&amp;\rm \text{tri-}&amp; 30&amp;\rm \text{triaconta-}&amp;300&amp;\rm \text{tricta-}&amp;3000&amp;\rm \text{trilia-} \\\hline
    4&amp;\rm \text{tetra-}&amp;40&amp;\rm \text{tetraconta-}&amp;400&amp;\rm \text{tetracta-}&amp;4000&amp;\rm \text{tetralia-} \\\hline
    5&amp;\rm \text{penta-}&amp;50&amp;\rm \text{pentaconta-}&amp;500&amp;\rm \text{pentacta-}&amp;5000&amp;\rm \text{pentalia-}\\\hline
    6&amp;\rm \text{hexa-}&amp;60&amp;\rm \text{hexaconta-}&amp;600&amp;\rm \text{hexacta-}&amp;6000&amp;\rm \text{hexalia-}\\\hline
    7&amp;\rm \text{hepta-}&amp;70&amp;\rm \text{heptaconta-}&amp;700&amp;\rm \text{heptacta-}&amp;7000&amp;\rm \text{heptalia-}\\\hline
    8&amp;\rm \text{octa-}&amp;80&amp;\rm \text{octaconta-}&amp;800&amp;\rm \text{ octacta-}&amp;8000&amp;\rm \text{octalia-}\\\hline
    9&amp;\rm \text{nona-}&amp;90&amp;\rm \text{nonaconta-}&amp;900&amp;\rm \text{nonacta-}&amp;9000&amp;\rm \text{nonalia-}\\\hline
\end{array}


So, $\ce{C_205H_412}$ is the molecular formula of pentadictane. $\ce{C_7547H_15096}$ is heptatetracontapentactaheptaliane.