In the late 70s and early 80s, I began my collection of metal minis with a box of PC types and 2 boxes of monsters. No matter what the players played or faced off against, those minis were what I used.
As we closed in on the mid 80s, I expanded my collection slowly. I added some plastic figures I found in toy sets. I bought some more metal figures (with paper route money) for important monsters the PCs would be facing. I bought a collection of leads I found at a garage sale. By the time 2nd edition was released, I had about 100 metal figures and 50 other figures.
As second edition was released, I entered high school and took on higher paying jobs ... which allowed me to purchase more equipment for the game. Books, magazines, boxed sets and modules were the majority of my purchases, but I continued to add figures at a steady rate when needed. By this point, if I needed a monster, I usually had *something* that could play the part pretty well. 500 metal figures, 200 plastic.
When I went to college, I left my figures behind. Instead of running games, I was a player most of the time. I brought a few PC types with me for use in games, but for the most part, my collection stagnated. I even gave away a bunch of my figures to other DMs.
Then, beginning in law school, I returned to my DMing as 3.0 was released. Healthy positions and a loving and supportive wife allowed me to increase my collection at an alarming rate. EBay and other online resources allowed me to find nice metal collections for very cheap prices. To go with all my figures from my youth, I added armies of common creatures (skeletons, orcs, goblins, etc ...) and those oddball creatures that you run into once every 4 or 5 years. By the time DDM was released last winter, I was up to 2,000 metal figures and about 300 plastic figures.
Since DDM was released, my collection has only grown by a few lead figures, but I have carefully crafted my DDM purchases to fill in many of the gaps I used to have in my armies. I buy a few boosters and trade at maxminis to get those select rares I want, focusing on the monsters that fill out my ranks of PC foes and monstrous allies. I use sites that sell singles to get those common armies I want (dretches, abyssal maws, etc ...)
Now my collection is well over 2000 metal figures and 500 plastic/vinyl figures. It will continue to grow, slowly, but I've pretty much reached critical mass in most of the common areas. Many figures have moved from the display book cases to the den shelves to make room for better designs (or better paint jobs). I expect to collect a lot more DDM monsters from future expansions and trade for a few more of the older expansion figures, but for the most part, I'm happy with where I'm standing right now.
So, how many do you need? You can get by with a few dozen rather easily.
How many can you use? There are no limits ...