VBA Macro: Paste underneath last cell in column with variable range - excel

Question

Title isn't the best so here a an overview.
I'm using VBA to copy select columns from one workbook to another, as what will be part of a larger automated program.
On the Workbook I am copying from, there are different sheets containing a "Stock Number" column. When pasting into my other workbook, I am trying to get these columns to merge into 1 single column (pasting below the last entry from the first sheet and so on).
Here is my current code:
Sub import_adam_article()
Windows( _
"copyfrom.xlsx" _
).Activate
Columns("F:G").Select
Selection.Copy
Windows("pasteinto.xlsx").Activate
Columns("A:A").Select
ActiveSheet.Paste
Windows( _
"copyfrom.xlsx" _
).Activate
Columns("N:N").Select
Application.CutCopyMode = False
Selection.Copy
Windows("pasteinto.xlsx").Activate
Columns("C:C").Select
ActiveSheet.Paste
Rows("1:1").Select
Selection.Delete Shift:=xlUp
NextRow = Range("A1").End(xlDown).Row + 1
Windows( _
"copyfrom.xlsx" _
).Activate
Columns("F:G").Select
Selection.Copy
Windows("pasteinto.xlsx").Activate
Range("A" & (NextRow)).Select
ActiveSheet.Paste
[A:C].Select
With Selection
.NumberFormat = "General"
.Value = .Value
End With
End Sub
The difficulty is that the amount of Stock Numbers will change every new file that comes through, so it needs to be able to adjust to varying amounts.
I can't seem to find a way to make it work and I've tried searching for answers elsewhere.
EDIT: The current issue with the code that it is selecting the next empty row to paste into, but only that cell, not a variable length down as required by the copyfrom column.