There is no such maximum limit.
The answer to this question is yes. Government employees or officers can be part of NGOs provided the NGO is not anti-government. There are a few rules too that these people have to follow, one of them is to make sure that the NGO is not profit making and the member must not draw any salary from the NGO.
The other alternatives are to start a Trust or a Society based on the objectives of your NGO.
The property of the company vests in the name of the Company and the same can be sold as per the rules mentioned under the Companies Act, (Eg: With the consent of the Board of Directors in the form of a resolution).