If I counted correctly, the editors of the 27th edition of this version of the Greek New Testament, known as the 27th edition of the Nestle-Aland,
consulted 586 Greek New Testament manuscripts, of which at least 291 were made before AD 800, and at least 35 before 300. It's "at least" because several of those manuscripts are dated 8th or 9th century, and several are dated around 300, or 3rd or 4th century. There are thousands of other Greek New Testaments available to scholars, but these editors -- Erwin Nestle, Barbara and Kurt Aland, Johannes Karavidopoulos, Carlo Martini and Bruce Metzger -- were satisfied with these 586. However, in addition to the Greek manuscripts, they also looked at 62 Latin New Testament manuscripts, at least 44 of those older than AD 800. The current location and catalog number of each of those 586 Greek and 62 Latin manuscripts is given, so that you can look them up or find photos of them, and look at exactly what the editors were looking at when they prepared this edition. They also consulted editions (that is, printed versions) of the New Testament in Latin, Syriac, Coptic, Armenian, Georgian, Gothic, Ethiopian and Old Church Slavonic.
And in the lists of these sources they have assigned a symbol to each one -- for example, p40, 2298 and d -- and in the so-called "critical apparatus" (I love that term), which is the strange stuff at the bottom of each page below the main text, they indicate which part of their text is supported by p40, or 2298, and so on -- and also indicate which manuscripts contain some other version of the text which they consider significant. (p40 comes from a fairly standardized list of New Testament papyri, from p1 into the p120's and still counting. I assume that 2298 is from some list of other New Testament manuscripts running into I don't know how many thousands. If I knew where that entire list was I'd tell you. I bet Bart Ehrman knows.)
And the editors of series like Oxford Classical Texts
or the Bibliotheca Scriptorum Graecorum et Romanorum Teubneriana (that's Latin for Teubner's Library of Greek and Roman Works)
do the same in each volume: provide a list of all the manuscripts and other sources they have consulted in preparing their texts, with a symbol for each one (Usually each symbol is a capitol letter because usually less than 26 manuscripts are used for a given text. But in cases of authors like Vergil or Terrence, editors might run out of capital letters, and also use small letters, and/or Greek letters, and/or numbers or abbreviated words or what have you.), and then at the bottom of each page they indicate which sources have the same text as the one they've chosen, and indicate other versions, which they consider significant, from other sources. In addition to these major variations, the Nestle-Aland provides dozens of pages' worth of minor variations at the end of the volume. In the Oxford Classical Texts and the Teubneriana and other editions of ancient works, such as this edition of the New Testament, the editors typically describe the manuscripts they've used, and in a case like this where there are more existing manuscripts besides the ones used, they'll give their reasons for using these ones and not those, and so forth.
They show their work when editing Sallust or the Bible, is what I'm getting at. It's usually not the same guys editing the Classics and the Bible, but the techniques are similar. Classics or the Bible, it's known as scholarly editing. And so while you or I might reasonably disagree with what Bruce Metzger said about how it's certain that Jesus existed, if we're going to criticize what he said about Biblical manuscripts and how the text of the Bible changed over the centuries, we better come correct, cause he was all up in it.