Indian investors have put in shares constituting about 21 per cent capital of Associated Cement Company Limited in response to a combined open offer by Gujarat Ambuja Cement Ltd and Holcim Ltd which closed today.   The response to the open offer of 21 per cent plus about 14 per cent stake already held by Ambuja Cement India Ltd (ACIL), a company through which duo had made offer, would increase acquirers holding to over 35 per cent, short of intended 50.1 per cent stake in ACC Ltd.   The offer has just closed and preliminary data indicates investors have responded to offer for Rs 370 per share of ACC by puting in over 21 per cent of capital, merchant banking sources associated with the offer said today.   Life Insurance Corporation of India, a siginificant institutional investors, has put in five per cent in the open offer, they said.