I think that the answer depends on many factors. I think that the answer to your question may be very different depending on whether the organization has a niche market or needs a mass market.

I know of a small organization...selling a limited product line...with only a few hundred Internet users in their constituency. But they created an inexpensive e-commerce site, meaning that selling a limited number of items still makes the e-commerce system worthwhile. 

How big is your organization? What are your sales goals? What are the characteristics of your customers, prospects, or other stakeholders?  What kind of e-commerce platform do you want/need to use?

All of these questions must be answered as part of your cost-benefit analysis.

thanks Michael for the input,

in fact and to be more specific, I am investigating this issue on national level. when ecommerce is mentioned people  mostly polititians  always associated it certain technology penetration threshold, like for instance Internet is used by 30% enterprises, and 50% household.\

It all depends on the scope of your e-commerce project. Also, the efficiency of technology is largely dependent on its users rather than the hardware side of the technology. Technology is only a tool and what you should rather focus on is your processes.

A well designed system of processes should help determine an appropriate answer to your question. 

Technology penetration threshold is not the only criterion for a successful e-commerce enterprise. Most often, in developing countries in particular, local culture and customs, laws and trust in the system (political and legal) are more prevalent. I am aware that politicians always feel that these are not important, but e-commerce has worked very well in African countries where technology was at its lowest levels, while it was considerably less successful in Asian countries where the local culture, customs and religion have had a restrictive impact.

ICT penetration per se is becoming less of an issue with the development of mobile internet (smart phones + 3G/4G). More important are the issues of the payment system and of logistics (I take it you are talking of physical goods). 

Actually you could probably have some form of e-commerce over 2G with e.g. M-Pesa.

But there are also other issues, as already pointed out, around trust and effective consumer protection.

Maybe you can find information in The Global Information Technology Report (2015 available), this report features the latest iteration of the Networked Readiness Index (NRI), which assesses the factors, policies and institutions that enable a country to fully leverage information and communication technologies (ICTs) for increased competitiveness and well-being.

To complete the information maybe you can check The Global Retail E-Commerce Index, a study designed to help retailers devise successful global online retail strategies and identify market investment opportunities, also the index  highlights the big and the small markets: the countries that are always going to be e-commerce behemoths because of their size, and the smaller yet still-promising markets where potential matters more than size.

Then you can compare the top 10 countries  in both indexes and find something in common about some factors, policies, education, I+D...

If you consider the success of Econometric Society Conference websites as a benchmark due to the Central Limit Theorem then you can use my paper titled ".....Econometric Society Conference case" on my RG page to give the average frequency for ICT penetration. SKM QC

E-commerce will drive Core Competency to optimum level; and Satisfactory Performance will facilitate access to variety of markets, where the consumer perception senses the cutting edge as they get better satisfied on the needs

http://icesba.eu/RePEc/icb/wpaper/ICESBA2017_ANDRONIE_P27-33.pdf

"Minimum ICT penetration threshold" among your target consumer and how to harness technology in compelling ways for online marketing of your product/commodity optimized with customer service support on eCommerce platform might be the right question here.

In my opinion, the minimum threshold, ie the required determinants for the development of online information and transaction services, is primarily a well-developed Internet infrastructure and the equipment of offices and institutions with computer equipment allowing effective use of information and other internet services. In addition, developed e-banking is indispensable for the development of e-commerce, e-commerce, e-logistics, etc.

In addition, the correspondingly higher levels of development of Internet information services in a given country is determined by whether the economy of a given country is classified as a knowledge-based economy and whether there are developed technological branches of the economy based, among others, on the development of innovative startups operating in the field of internet information services and other enterprises economic and research and development of information technologies typical of the current technological revolution, known as Industry 4.0.

The key issue is also the scale of state expenditures, ie funds allocated from the state budget for the development of the Internet infrastructure in the country, including offices and institutions of state administration. In some countries, the scale of these expenditures is high for at least a few years and in these countries the majority of official matters are citizens, including entrepreneurs remotely, via the Internet. In some countries, the electronisation of citizen services within e-government as well as universally operating standards of electronic identity cards, electronic signatures, electronic banking, etc. is developed.