Distribution wasn't the best it seems, spotty at best, it seems during that time. At least it was that way in the area I was raised in.
It occasionally seems to have taken nosedives, too. I can remember books--even semi-major titles, like Green Lantern and the like--failing to make it to the shelves as late as the mid-80s.
I also had a peculiar run-in with clerks, by the way. In my case, a magazine shop that was selling used comics, over the course of a month, somehow metamorphosized into an adult bookshop. In itself, that might have been mildly awkward, but they hadn't changed the signs, so I wandered the store for a good ten minutes (I couldn't have been more than fifteen at the time) before asking for help and getting panicked shouting answers from clerks who apparently didn't notice me walk in. They also denied having ever sold comics...
Interestingly, that shop is still open (and selling adult videos) after...hmm...more years than I care to think about. No, I don't shop there. I pass it daily on my commute.
Oh, right. Counting titles. If it seems like I'm teaching a lecture, it's not condescension, but rather me not ever wanting to do this again...
You'll need a few ingredients. The GCD is a big one. You'll also need Firefox with the Table2Clipboard add-on (a favorite toy of mine). Lastly, Microsoft Excel will be very, very handy. (Or, if you're just happy with a single number of titles printed in 1952, jump to the end and save yourself some fairly dry reading.)
I hit the GCD's advanced search, and asked for Issues (rather than Stories, the default), Starting with 1/1/1952 and Ending 12/31/1952, published in the United States and in English.
If anybody with influence over the searching mechanism happens to be reading, I'd really prefer the issues to be the default, a more obvious way to use the date (even a single example to show that "1952" only turns up a dozen or so books), and maybe a checkmark to say, "yes, I want everything at once instead of chopping into result pages." But those are minor nitpicks in the grand scheme.
Anyway, click "Search" and start harvesting. On each page, right-click the table. If you installed Table2Clipboard, you'll see it in the context menu that pops up, and from there, you can "Copy Whole Table." Do so, then paste it into Excel, then (with Ctrl-End and some minor arrowing) get ready for the next page. Lather, rinse, repeat.
Now I have an awkward spreadsheet with every book published in 1952 in Excel. First thing, select all the data (Ctrl-A) and if you're an old fogey like me that still uses menus (i.e., I can't help you if you're using Office 2007 or something, with the "ribbon" up top), Edit, Clear, Formats, gets you something more readable.
Then for some minor magic tricks. Thankfully, I don't care about the Date column, because it's about to get trashed. Select the Issue column (C in mine), and click Tools, Text to Columns. Delimited text, split on "Other," an open parenthesis. Since the GCD names the books as "Title (year series) #nn," this gives us a clean series title. Try to ignore the little flags on the left. I have no idea how to select them en masse to delete them, but they're harmless...
Now, sort (Data, Sort) on the column that has the titles. And then run down the list and delete all those headers you copied in--they'll sort together, to make it easy to nuke the rows at once.
We're almost there, I promise, and here's the neat part. Select your data again, plus one more column to the right. Now hit the menus for Data, Subtotals, which I want to know why nobody told me about it! At each change in your (sorted) title name column, tell it to Count (not that it really makes a difference) and put the subtotal into (at least) the empty column. Excel will then busily insert bolded rows with your title and...well, the number of things in the empty column for that title, so zero.
But that's OK, because we don't care how many issues of Frogman Comics we have for the year (if you did, put the subtotal in an otherwise-used column), but rather how many titles. Which means we need to count those zeroes. So...go down to the lower-right of your data (Ctrl-End), and type something LIKE the following:
=COUNT(F2:F2153)
(The equals sign tells Excel to get to work. Count is self-explanatory. F is the column where I put the subtotals. I start at row #2, because the headers are in the first row, and end at row #2153, because that's the row where the final subtotal (for Young Romance) is sitting.
If all went well, it should only take a few minutes (it took me more time to explain this than do it), and you'll get an answer.
For 1952, I got a fairly large-sounding 418 titles. On the other hand, I know that, today, companies track the top-300 in sales, so that means the market must be far more crowded.
(Trickier would be to have the spreadsheet screen out books that only got an issue or two out, but I'm waaay too lazy to deal with that. One approach would be to take a recent GCD dump, load it into a database system, and write some SQL, which would get that answer in one step. But I also realize that's not exactly appealing to the average home user.)
