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Hi,
recently I bought this type of UK Stamps



I use a4 envelope and the weight is around 0.30-0.50 grams
how many stamps I have to put on the envelope ?

If you use http://www.royalmail.com/portal/rm/cont ... tId=400036 you can check the tables for prices in future

For an A4 envelope, the prices are as follows:

20-40 grams £1.35
40-60 grams £1.82

Those stamps (called "second class") are worth 30p therefore you'll need 5 or 7 of them (you'll need to check the exact weight and see if it's 20-40 or 40-60).