It depends! Let's say a 8% Treasury exists and you want to buy it today. To establish its price, you need to know:

• What is today's yield curve? (i.e., what is the market's expectation for interest rates at different points in the future?)
• When does the bond mature? (i.e., how long until the face value of the bond is paid out back to the bond holder?)
• How frequently does the bond pay interest payments (coupons)?

I've put together a quick calc based on Federal Reserve yield curve data as at 27 Sept, assuming an 8% Treasury maturing in exactly 20 years, with semi-annual coupons (as most government debt is semi-annual). Google sheet calc

If you bought $3m worth of this fictional bond today, you would own $1.95m notional of the bond. You paid $3m for $1.95m of US gov't debt effectively because the bond was issued in the past at a higher yield that what the market is expecting the government to issue bonds at in the future.

Every 6 months, you would receive a coupon of c. $78,000, or effectively $13,000 per month. This is interest the gov't pays you for having lent it money (or rather having bought the debt from whoever lent it money.) These payments are guaranteed as long as the US gov't remains solvent.

Finally, in 20 years' time, you would also receive the principal payment of $1.95m. This is the government paying back the amount it originally borrowed. Note that it will likely be worth significantly less in real terms in 20 years!

Importantly, you don't have to hold the bond to maturity and wait 20 years to get your $1.95m. Just like you bought the bond at a bid price of $3m today because rates are lower than the coupon yield of the bond, if the yield curve decreases further, the price of your bond in the open market will increase. E.g., if yields went down 1% across the curve, your $3m investment would now be worth $3.4m and you could sell it for a tidy $400k profit!