How Excel Understand Numeric Number Entered In Any Cell

I have Two Sheets in same workbokk.

Sheet1 Sheet2

Sheet1 B2 = "any numeric value (whole digit i.e 10 or 55 like) - user input "

Sheet2 Starting point is cell B3.

What need to happen - As soon as user enter number in Sheet1 B2 = any value say, 20 then,

Sheet2.Select
Range("B3").select

suppose i assign value p as,

dim p
sheets("sheet1").select
p = range("B3).value

now here i want to indetify this number, i.e.- 20 (User input) and then as soon as user enter this value,

step 1 - sheet2 get select.

step 2 - from B3 sheet2, by row wise it select next 20 row as well next 20 columns.

for eg.- from B3, the data should select like following,
B3 to U22.

and this selected area highlight with single border only. That's it.Therefore, my main problem is how excel will understand this numeric number ?