it depends on the gravity of the wort you are pitching into finally. Generally if you are stepping up a starter you want to pitch the tube of yeast into say 1 qt (or litre if you prefer) of 1.030 wort and when that is complete cold crash, decant the liquid and add another 1 or 2 quarts, complete, crash, decant, repeat.
That being said I think that if you need that much yeast it might make sense to brew a 5 gallon batch of 1.030-1.040 beer and use the eyast cake from that. Unless your making lagers in which case someone else will have to chime in as I have not done one of those.