Many files, if you intend a total porting of the faction. First of all, there is no free faction slot in vanilla RTW, which mens you'd have to replace one of the existing factions.

This means: new units (export_descr_unit, descr_model_battle, export_descr_buildings, export_units, UI cards, export_descr_sounds_unit_voices [maybe some otehr sound files]), new faction name (expanded_bi, strat, other files in the text/ folder, there's a guide by Epistolary Richard), new faction banners (lots of .tga files here and there, there's a guide by Monkwarrior on this), new buildings (export_desc_buildings)

Also, some aspects of the game are culture-specific (portraits, music, building cards, interface, etc) which means you'd have to insert a 7th culture in the game (several files, there's a guide on how to do that by Dol Guldur).

I suggest you strat with implementing a new culture first. Then assign this culture to the faction you're gonna replace (say Britons). Then rename the faction. Then start removing the units and putting in your own. And then continue wokring step by step, following the tutorials you can find here and at TWC.