The phrase "add the same amount" is correct and usable in written English.
It can be used when instructing someone to increase a quantity by a consistent value across different instances or items.
Example: "To maintain balance in the equation, you need to add the same amount to both sides."
Alternatives: "increase by the same value" or "add an equal quantity".
If using dried beans, add water to cover and then add the same amount again.
Measure the total amount of egg in cups, then add the same amount of whole milk.
Iran is expected to add the same amount again within six months, bringing its crude exports back to pre-sanctions level.
Add the same amount of water, cover with foil and turn the oven down to 150ºC/Gas 2 for 1 hour.
The other needs a little more advance preparation – I tip a couple of tablespoons of the beef fat from the joint into a small pan, heat, and then add the same amount of flour and continue stirring and cooking for two or three minutes, until it has just started to colour.
The hat will then be given to Player II and Player II can look at what's in the hat and either she can match what's in the hat, i.e., add the same amount in so 1 if it's 1, or 3 if it's 3, or she can simply take the cash.
Every new member adds the same amount of value, no matter how large the network becomes.
Kino poured some White Label into a glass, added the same amount of water and two small, nicely formed ice cubes.
In comparison, scientists added the same amount of research over the first four years of the network's operation.
To get different dilutions of the antibody, I added the same amount of antibody to increasingly large amounts of the diluting solution.
For both substrates, adding the same amount of MA to untreated samples (as a feed additive) resulted in greater CH4 production and ammonia-N concentrations compared with the MAH treatment.
