A person doesn't have to witness a miracle in order to have sufficient reason to believe that one occurred. But how many people in the modern world claim to have witnessed a miracle?
Numbers are often hard to come by, and the quality of the information can vary from one circumstance to another. We often have to piece things together and make rough estimates. But there's enough information to go by to lead us to some significant conclusions.
Craig Keener cites a large variety of sources that give us numbers for different groups, and he often refers to hundreds of millions of miracle witnesses (Miracles [Grand Rapids, Michigan: Baker Academic, 2011], 762). One survey found that 48 percent of people in the United States claimed to have witnessed at least one miracle (238). Even among noncharismatic Christians alone in the United States, more than a quarter claim to have witnessed a healing (505-506). Surveys and other sources have found that half or more of converts to Christianity in China claim healing, their own healing or somebody else's, as one of the reasons why they converted (264, 297, 300-302). Keener notes that "In sub-Saharan Africa as a whole, 56 percent of Christians claim to have witnessed or experienced divine healing." (313) He cites data showing that most doctors claim to have witnessed one or more miracles among their patients (427-428, 721). In some countries, half or more of Christians claim to have witnessed an exorcism (813). And so on. He cites far more information than I can repeat here. Though such numbers tend to be higher among charismatics, Keener argues at length for percentages well into the double digits even among noncharismatics. Among Pentecostals and charismatics alone, he estimates the number who claim to have witnessed a healing at three hundred million (238). He estimates that more than a third of Christians who are neither Pentecostal nor charismatic claim to have witnessed at least one healing (239). And healings are just one type of miracle. When other miracles are included, and non-Christians are added to the total, the number has to be well beyond a billion. I would expect it to be in the multiple billions.