|A couple of questions...|
1 - Are you confusing Rows and Columns?
In your first sentence you say you want to "add a value to 1 row depending on the value in the first"
In your next sentence you say "i need to put a certain number in the cell next to it"
Next to it implies Columns, above it or below it would imply Rows. Which is it?
2 - "its about 20 numbers that require different values in the cell next to them"
Is it safe to assume that each of the 20 numbers has a unique value associated with it?
If that's the case, and assuming the first list is in a column, you should be able to use VLOOKUP. Look up the VLOOKUP function in the Excel help files for a full explanation of how it works.
Create a table someplace in your spreadsheet with your 20 numbers in one column and their associated values in the column next to it.
Let's say the table is in S1:T20
Now let's say your original numbers start in A1.
In B1 enter this formula as drag it down. Make sure you use the dollar signs ($) as indicated:
This will look up the number in A1 in the table in S1:T20 and return the associated value from the second column (2) of that table. As you.drag it down, the A1 will increment to B1, C1, etc. but the $S$1:$T$20 will remain constant because of the $.