I'll ditto the seagull. 数多 has probably the widest use in written work (at least in my office), but even though takusan is almost always written in kana, 沢山 is the correct kanji. The reason, or at least so I have been told by my colleagues is that 数多くis more polite and should be used for writing, while 沢山 is normally reserved for speaking only! But, again it is context and as always in Nihongo, there are special exceptions because they, of course, are special people.
"There are those that learn by reading. Then a few who learn by observation. The rest have to piss on an electric fence and find out for themselves!"- Will Rogers