Something in the range of 2000 - 5000 iu to ensure immune protection. That's 50 - 125 µg (micrograms). How much you want to take will depend on your:
age (older people make less vitamin D in the skin)
skin tone (people with darker skin take longer to produce vitamin D under the same sun conditions)
having a high BMI (vitamin D is absorbed and sequestered into the adipose tissue before it is available in the blood for immunity)
latitude (countries further away from the equator get less sun)
diet and lifestyle (some micronutrients and cholesterol are required to manufacture/convert vitamin D in the body... plus how much summer sun you get and how much clothing your wear)
Ideally, start off getting a vitamin D blood test, you can get one direct to the public from the NHS labs in Sandwell (google it). It's a fairly modest £30 or so.
If not, download the DMinder app, which can track your vitamin D intake from supplements, food and sunshine and give you an estimate of your blood levels. I think it might be free at the moment, otherwise it's a great value £2 from the app store.
Remember to ensure you get sources of vitamin K2 and magnesium in your diet if you take higher amounts of D3 (eg 5000 iu +), as they work synergistically together to ensure good immune, bone and artery health and correct metabolism of calcium. K2 helps shepherd calcium to where it should be in the bones & teeth and away from the soft tissues and arteries.
Talk to your doctor if you have any calcium related conditions like kidney stones, as hypercalcemia can be a risk if taking very high doses (K2 does mitigate this).