I have searched a number of websites and articles relating to nomenclatures or phages but never found anything explaining what T in T-4,5,6.. Phages mean in the Coliphages. Also, what does the number mean? I have seen that many times, the T - even are grouped as one, why is it so? Thank you.
The "T" stands for "Type" as in type 1, type 2 etc. To quote Wikipedia on Escherichia virus T4 (emphasis mine):
Bacteriophages were first discovered by the English scientist Frederick Twort in 1915 and Félix d'Hérelle in 1917. In the late 1930s, T. L. Rakieten proposed either a mixture of raw sewerage or a lysate from E. coli infected with raw sewerage to the two researchers Milislav Demerec and Ugo Fano. These two researchers isolated T3, T4, T5, and T6 from E.coli. Also, in 1932, the researcher J. Bronfenbrenner had studied and worked on the T2 phage, at which the T2 phage was isolated from the virus. This isolation was made from a fecal material rather than from sewerage. At any rate, Max Delbrück was involved in the discovery of the T even phages.
His part was naming the bacteriophages into Type 1(T1), Type 2 (T2), Type 3 (T3), etc.
It turns out Wikipedia is incorrect: The original use of Type as a designation for these viruses comes from a paper published in 1945 (PDF) by M Demeric and U Fano: BACTERIOPHAGE-RESISTANT MUTANTS IN ESCHERICHIA COLI Genetics 30:119. In this paper they collected phages from Luria (of LB/Luria Broth (actually properly known as lysogeny broth)) and Delbruck amongst others and collated them all for comparison. Delbruck references them in his review article (PDF, possible paywall, summary here), also from 1945.
I could find no evidence for why the phages were numbered as they were, though strains from Luria (actually previously from his supervisor as P28 and PC) were labeled T1 (Luria alpha) and T2 (Luria gamma), so it is possible that it was simply in order that they were received or possibly in order of discovery amongst the several groups working on these viruses.
The T-even phages were grouped because of structural similarities, and are now strains of the species escherichia virus T4 in the genus Tequatrovirus (see link for taxonomy).