It is clear from the books that the Olivander family has been in business for a very long time (since 360 BC is quoted) so it is ot unreasonable that they will have a lot of stock.
Similarly as Olivander says in the first book 'the wand chooses the wizard' to it is reasonable to assume that a premium want shop would want a lot of excess stock to be able to offer the best possible choice. So even if he only sells a few hundred per year he might well have 10, 20 or 100 times that number on the shelves.
As well as the simple pragmatic demands of stock control Olivander is a wand 'maker' not just a seller, retailer or importer so it may well be that he sees himself as an artist and is driven to make the best wand he can from the materials which become available. Clearly wand materials like phoenix feathers (Fawkes only ever gave two) dragon heartstrings etc are hard to come by and so it's not at all hard o imagine that Mr Olivander sees each wand as an individual work of art and as such he may not be at all bothered whether he sells any particular wand in his own lifetime.
With the above in mind it may be more significant to ask about how many wand he could make per year. After all just 100 per year is 1 wand every 3.65 days with no days off which is pretty fast for an heirloom object considering he also has to look after the shop and lets say that sourcing the materials takes a bit of effort as well.
There is also a line:
Arthur Weasley: "Shop's empty. No sign of a struggle. No one knows whether he left voluntarily or was kidnapped."
Ginny Weasley: "But what'll people do for wands?"
Remus Lupin: "They'll make do with other makers. But Olivander was the best, and if the other side have got him it's not so good for us."
This implies that he is not the sole supplier in Britain but certainly regarded as the best.
To me this suggests that Olivander's stock in his shop represents a lifetime's work plus perhaps that of his ancestors as well.
Also perhaps some wizard make do with (or prefer) other domestic or imported wands. SO he may not be supplying the entire Hogwarts intake every year.
If is also not unreasonable to assume that at least some wands are inherited and there seem to be no cases in the canon where wizards routinely own more than one wand (with a few exceptions where wands are temporarily taken from their owners as in the assault on Gringots).
It seems pretty clear that Olivander does make wands and that he is an expert on the subject. what is less clear is whether he tops up his stock with wands from other makers...for example is there an Olivenders 'extra value brand' imported from China ?
We do know of Gregorovitch as one other wand maker who supplies at least one Durmstrang student (Viktor Krum) and we might guess that he is the Bulgarian or Eastern European equivalent of Olvander, although it is never spelled out where, for example Beauxbatons students prefer to buy their wands. It may be that every country in europe has their own wand maker(s) or there may just be two or three centres of wand making.
