Before Ottmar Mergenthaler (1886) and Tolbert Lanston (1885) made their inventions enabling printers to create their own metal type, if a printer needed more type they’d send round to the nearest type foundry. Of course for many printers this went on for years after these machine setting innovations. Well into the twentieth century, George Bernard Shaw was among the conservative authors who insisted that mechanical typesetting machines like the Linotype and the Monotype should never be used for his books. He wanted everything set by hand. And this meant a trip (or several) to the type foundry to get hold of the types you needed.
If you buy a font (font in America, fount in Britain) of type how many “A”s or “B”s will you get?
Wikipedia informs us that “A font when bought new would often be sold as (for example in a Roman alphabet) 12pt 14A 34a, meaning that it would be a size 12-point font containing 14 uppercase ‘A’s, and 34 lowercase ‘a’s.” The number of the other letters followed from that in some regular proportion governed by the frequency of use of that character in the local language. You are after all buying an amount of metal, an alloy of lead, tin and antimony, so the numbers of each character will vary according to size and other features. Here is the Font Schemes Chart from Skyline Type Foundry in Prescott, Arizona. They tell you how to use it in the bottom right hand corner. (I don’t know how much, if any, variation there might be between the counts for different foundries.)

What this means is that (if they work from a chart using the same proportions) if you bought a font of 12 point Binny Old Style No.21E, 15A 32a, (shown below) from M & H type foundry in San Francisco, you’d be getting 32 of the lower case a; 13 of b; 17 of c; 19 of d; 43 of e (our commonest letter); 17 of f; 13 of g; 21 of h; 32 of I; 9 of j and k; 21 of l; 17 of m; 32 of n and o; 13 of p; 6 of q; 32 each of r, s, and t; 17 of u; 9 of v; 13 of w; 6 of x; 13 of y; and 6 of z. You can work out the punctuation marks that’d come along with the font.

This font might do you for surprisingly little. In fact you’d already need a second font before you’d finished setting the first eight lines of the page below from Plays Pleasant and Unpleasant by the author described on the title page of my possibly pirated American edition of 1904 as Bernard Shaw. (The book is of course not set in 12pt Binny Old Style, so all this pretended precision is just approximation.) This copy seems to be a reprint, as the publisher, Herbert S. Stone and Company of Chicago and New York audaciously claims “Copyright 1898” in their own name. Thus these considerations are irrelevant, as the book was no doubt set without the author’s involvement, using the Linotype machine. Had it been handset, the letter we would have run out of first was lower case “w”, but it matters not which one it was, off to the foundry for another font. Running low would also be “t”, of which you’d only have five left.

Obviously you’d need quite a lot of bits of type to print a book. In addition to the Roman we’ve looked at, you’d need Italic, and Small Caps, as well as larger sizes for display lines, running heads and folios. In the early days of printing a sheet of however many pages (4, or 8 most probably) would be printed and set aside while the type was broken up (distributed) and used for the setting of the next few pages. And so on and so on until done.
Sheesh! Talk about labor-intensive. I remember when the last hot-type shop in Santa Monica (California) closed its doors in the mid ’70s. Ironically, it was called “Harold’s Type.” And then, after a few years, when all the typesetting companies that didn’t diversify to support “desktop publishing” also followed into closing down. By then, I was fully into collecting my digital font families.