How many forever stamps does it take to send a normal letter to England?

3.

Each US Forever stamp is 49 cents and sending a normal letter to any other country costs $1.20. So 3 US Forever stamps would cover that postage.

The USPS does sell a Global Forever stamp at $1.20 each if you want to go down to the post office and buy some.

I send international letters several times a month. That's how I know this stuff.

Check the Postal Service web site. Generally, international mail must be sent with MONETARY DENOMINATION on the stamps. "Forever" stamps won't work.

I assume you are in the USA as you mention Forever stamps.

It's not a matter of how many stamps, it's a matter of how much postage. The USPS web site states that the price of a 1 oz letter to any other country is $1.15, therefore you would need three. But as that would cost you $1.47, you'd end up paying too much. If you're going near a post office soon, save a few cents and buy an actual stamp for $1.15 at the post office, and they can also give you anything else you need such as an airmail sticker to make sure it doesn't end up crossing the Atlantic by ship and taking weeks! You don't want that - $1.15 pays for it to go by air and that generally takes a week to arrive in my experience.

This stamp would in fact be a Global Forever stamp. These are round to make it obvious what they are.

A mail piece of an ounce or less to the UK, which is typical for letters. Prices begin at $1.15. That would mean three forever stamps, but you'd be wasting a small amount of money since the current price of three stamps would come to more than that.

have the letter weighed and size checked at your local post office and they will tell you postage to the UK.

The USPS has a website.