Space agencies are currently tracking thousands and thousands of bigger pieces of space debris left by old satellites and rockets boosters. This
tracked stuff with known orbits consists from way above centimeter sized objects, mostly over ten centimeters sized pieces, including also few hundred
old satellites, making impacts of these highly lethal to satellites and spacecrafts.
Nevertheless these numbers they aren't biggest threat to satellites and spacecrafts because that huge amount of space up there makes their density
very low, also capability to track these enables evasive manoeuvres before collision happens.
But of course debris with known orbits is just small part of total.
Most numerous debris pieces are dust particles, small meteoroids and things like that, also man has added his two bits to this, explosions of rockets
boosters produce (and spread) all kinds of debris very effectively.
What is common for this class is their size which is under one centimeter making their observing and tracking impossible.
Even this small objects could cause lot of damage to satellites but small size also enables protecting satellites against them because their kinetic
energies aren't enough to penetrate available shieldings. Although collisions with these can't be avoided they can't cause serious damage to well
shielded satellites itself, only to external parts like solar panels, in fact solar panels retrieved from Hubble on service flight have had small
numbers of visible holes and impact marks caused by these. (and lot of microscopic impacts)
Most dangerous class is one with object size between one centimeter and ten centimeters because they aren't so easily observable and their high
numbers makes tracking individual orbits hard. So as you can conclude yourself total amount of these can be only speculated, but nevertheless it's in
class of hundred thousand and more. Shielding satellites and spacecrafts against these would be extremely hard and require very heavy "armor" so
objects of this size are well capable to crippling or killing any satellite/spacecraft.
Fortunately there's enough space to make collision of these much rarer than you would think considering their numbers, it has been estimated that
collision between satellite/spacecraft and ten centimeter sized object happens once in decade.
ESA - Space debris: assessing the risk
2.5 mm hole in Hubble's solar panel
Impact "hole" caused by 1.2 cm Al sphere travelling at speed of 6.8 km/s. (target is 18 cm thick Al plate)
NASA Orbital debris FAQ
Cool Space Junk Facts
Long Duration Exposure Facility (LDEF)