A set of standard stainless steel pipes, each of internal diameter $26.65\:mm$ and $6000\:mm$ length, is used to make a plug flow reactor by joining them in series to carry out degradation of polyethylene. Seven such pipes are required to obtain a conversation of $66\%$ at $450\:K$. The minimum number of standard $8000\:mm$ long pipes of the same internal diameter to be procured for obtaining at least $66\%$ conversion under the same reaction condition is _______________.