Lose Focus!
In the field where the number is being entered into, in the LostFocus event, do something like this
dim db as Database
dim rs as Recordset
set DB = CurrentDB
set rs = db.OpenRecordset("Select Name from Customer where Number = " & txtNumber.value & ";")
txtName.value = rs.fields("Name")
Note: You must have a reference set to DAO in order to be able to declare a variable "as Database" or "as Recordset"
Cheers!