Evidence is, in a general sense, anything used to support an assertion. The standards used to determine what sort of evidence is acceptable can vary, depending on the situation.
What may qualify as evidence in a casual argument might not qualify in a formal debate, legal proceeding or scientific investigation. Hearsay, for example, is often used in casual conversation to support a claim, but it isn't permissible in most courtrooms. In science, evidence is usually expected to be empirically observable and repeatable. Additionally, if a piece of evidence implicates more than one cause, additional evidence should be presented to exclude the other possibilities. Scientific experimentation depends on the analysis of many observations in order to determine consistent patterns and to reduce elements of chance or uncertainty. Any claims, or hypotheses, derived from observations must be falsifiable if disconfirming evidence is observed.
When skeptics object to apologist's claims on the basis of insufficient evidence, they're generally referring to a lack of scientific evidence or a preponderance of anecdotal evidence. For any given claim, there exists a burden of proof which must be supported by evidence. The defining characteristic of the required evidence rests in its quality, not quantity.
If, for example, someone claims "God answered my prayer" that is both an assertion (of a specific occurrence) and evidence for the general assertions about the existence of God and the efficacy of prayer. Unfortunately, the quality of that evidence is exceptionally low. So low, in fact, that such a claim is only considered evidence in the very broad, definitional sense. Anecdotal evidence and bald assertions have no evidentiary value and serve only as additional claims.
Suppose that 2.5 million people all testified, "God answered my prayer". The sheer quantity of evidence is sufficient to warrant an investigation of the claims, but the testimonies still don't qualify as sufficient evidence to support acceptance of the claim. Increasing the quantity of claims doesn't increase the quality.
Apologists commonly make claims supported by anecdotal evidence, hoping that the sheer quantity will be sufficiently convincing. Unfortunately, this tactic is often effective and claims like, "80% of the population believes in God — they can't all be wrong" can be very convincing to those who don't critically examine the claim. This argument works because people are reluctant to write off that 80% as delusional, stupid or insane. (See Argumentum ad populum.)
The truth is that they most definitely can be wrong. At some point in human history, the overwhelming majority believed that the Earth was the center of the universe. That majority was wrong. They weren't necessarily delusional, stupid or insane; they were simply ignorant of the facts and wrong.