I would change

strNewFileName = ActiveSheet.Range("a1").Value

To

strNewFileName = "Number " & ActiveSheet.Range("a1").Value

Question 2 (you really should put your separate questions into separate
emails. Many times you will only get a partial answer or if someone
does not know the answer to both questions they will not answer either
question)

Add Error Handling to your code  to catch the error. Something like:

On Error GoTo EH

'Place procedure steps here.

'Handle Errors Gracefully
Exit_EH:
Exit Sub

EH:

Select Case Err.Number
Case 91 'Or whatever error you are getting
On Error Resume Next
Case Else
MsgBox Err.Number & ": " & Err.Description
Resume Exit_EH
End Select

For the Question 1;

I prefer Val() Function

if val(Range("A1").value) = 0 then ' if it consists a text (not a number), val
returns 0
'This is not a number
strNewFileName = Range("A1").value
else
'This is a number
strNewFileName = "Number" & Range("A1").Value
end if

But remember if Range("A1") consists a number  and the number is 0,then the "IF
STRUCTURE" will act like that it is a text. I mean it won't make strNewFileName
= "Number 0". You can make a little special solution for this. For example you
can control it like this:

if val(Range("A1").value) = 0 and not Range("A1").text = "0"
strNewFileName = Range("A1").value
else
strNewFileName = "Number" & Range("A1").Value
end if

So it fixes the 0(zero) problem. This If Structure will always put "Number" in
front of it, if the value is a number.