Ask your GP for a blood test to determine the level of Vitamin D3 in your blood.
The normal range should be somewhere between 125-150 nmol/l, but I understand that people with MS can often be deficient in this area. By exactly how much will vary from person to person, so your level ought to be checked first - BEFORE you consider whether supplementation is necessary and at what level !!
I’m sure others will be able to offer further (probably more precise and better) advice shortly?!
(PS: I just did my own ‘home based’ blood test - through ‘cityassays.org.uk’ and, having taken Vit D3 supplements of 10,000iu just because I thought I ought to in case, my result came back as 299nmol/l - so high that ‘cityassays’ phoned me and suggested I stop taking the supplements immediately.)