The number of participants or number of researches in a conference is not as important as the quality of these researches.
Some profitable organizers give much importance to the number of researches and take it as a Key Performance indicator to the success of the conference. Whereas, other non-for-profit organizers take the quality of researches as a Key Performance Indicator.
Apart all conditions and requirements you have already mentioned my opinion is that the hospitality should be at its maximum, in content, elegance, originality and spontaneity.
Beside many things already mentioned by my colleagues, dear @Amir, do not forget venue and social life of conference attendees! Many of us like to travel around, to visit local restaurants, historical and culture places around...! Successful Conference must fill all of those criteria!
The major advantage of a scientific conference is to get into direct contact with scholars who share your specialization & probably your interests. Let us face it: most of the conferences are monotonous with successive short talks (20-30 minutes) followed by quick relevant & irrelevant questioning & discussions.
Dr. Amir has raised a very good question. No one likes to travel a long distance to attend a boring conference with very limited advantage. To make the conference attractive, successful , and beneficial the organizers ought to prepare diverse activities besides the talks such as visits, some recreation, and some contests with prizes. The organizers ought to display generosity & hospitality for all the visiting scholars.
Some of the factors that might make any conference successful are inviting well-known speakers, having good main themes and planning some interesting activities.
put strategic plan for the future,,,, my first experience will decide if i will participate in the upcoming ones. so stick to time scheduled for start, opening ceremony, lectures, check set up early do not be reactive; projectors light, pointers, recruit committed chair persons to control and time allowed and discussion, do not print certificate of attendance prior to the conference. ,,,, workshop should be either before or after, i think 3days is too much two days is quit suitable. start morning lecture little pit late after 10 o'clock