It sounds like you need to use the vlookup function. It goes like this:
=vlookup(value, table range, col, type)
First I would give a name to the table with the rates (insert, name, define). The range name, whilst not absolutely necessary, makes reading the function a little clearer. Lets say you call that range "rates", and lets say that the number of emails you're looking up for company 1 is in cell b2 (on the company 1 sheet), then the formula looks like this:
=vlookup(b2,rates,2,false) - the word false can be left out - so =vlookup(b2,rates,2) will do the same. False tells excel to find the lowest closest match, true tells it to match exactly. Anyhow, the function will lookup the value in B2 in the first column of the "rates" table and return the value in the 2nd column. Change that 2 to a 3 and it will get the value in the third column i.e company 2 and so on. Hope it helps.