I think it's useful to distinguish between public and private miracles. That's a rough-cut distinction. There's some overlap.
i) By public miracles, I mean a miracle that's sufficiently impressive, as well as witnessed by enough people, that it has value in validating a religion. To serve that function, enough people must see it so that it takes on a legendary status. It becomes famous through word-of-mouth. Likewise, it helps to be a spectacular miracle.
The primary value of a public miracle is to authorize religion. But it could have beneficial side-effects. For instance, the ten plagues bore witness to Yahweh through the public humiliation of the Pharaoh cult and the gods of Egypt, but they were also instrumental in delivering the Israelites from bondage.
ii) By contrast, a private miracle is a miracle that God does for the benefit of an individual. It may only be known to that individual or a handful of people in his inner circle. Although it may bolster his personal faith, it's not on a scale sufficient to validate religion for second parties. The miracle is unknown to most outsiders.
The function of a private miracle may be an exercise of divine mercy. The design isn't to confirm or prove God's existence, although it might have that side effect for the beneficiary, but to help someone in need. Take a dramatic answer to prayer. Not prayer for a divine sign, but prayer to relieve an urgent or desperate extremity which only God can meet.
Or a private miracle might be for the benefit, not of the immediate recipient, but someone further down the line, in a chain reaction. Say the miracle is to benefit the great-grandson of the recipient–who won't exist apart from a miracle upstream to himself.
iii) We should distinguish between the ontology and epistemology of miracles. To function as a divine sign, attesting religion, a miracle must be recognizably miraculous. But in principle, an event could be miraculous even though people fail to recognize the miraculous nature of the event. What makes it miraculous is the kind of event, and not how it's perceived.
To take a comparison, suppose a used-car salesman turns back the odometer on every car he retails so that no car displays more than 50,000 miles. Even though that's his uniform policy, there's something funny going on, since it's highly unlikely that every used car will naturally have such low total milage. Someone had to monkey with each odometer to produce that result. In this case, uniformity is suspicious.
iv) Apropos (iii), in principle, private miracles could be frequent. But because private miracles are isolated events which happen to ordinary individuals, they are consistent with the apparent rarity of miracles. Since, in the nature of the case, private miracles aren't well-known, even if they were common, their frequency wouldn't diminish the value of public miracles, since most folks would remain ignorant of all, or nearly all, private miracles. Public miracles would still stand out against the apparent regularity of nature. Miracles in the public domain could be infrequent while miracles in the private domain could be frequent. I'm not saying that's the case in reality, but it's a useful clarification.
Likewise, private miracles might be more prevalent at a particular time and place, but less prevalent at other times and places. Or one individual might experience several miracles in the course of a lifetime while another individual might experience none. That would depend on factors like persecution, inaccess to mundane solutions, and the strategic placement of miracles to further God's agenda in history.