Simple VBA macro to split a text file (*.txt) into chunks where maximum number of lines for each chunk is entered by user. The output files get the same name as the source text file, but with _1, _2,... appended to the file name (example.txt => example_1.txt, example_2.txt etc.).

fileSplit

Sub split()
    Dim fNameAndPath As Variant
    Dim TextLine, fName
    Dim fileCount, myCount, numberOfLines As Integer
    Dim outputFile
    
    myCount = 0
    fileCount = 1
    
    fNameAndPath = Application.GetOpenFilename(FileFilter:="Text files (*.txt), *.txt", Title:="Select File To Be Opened")
    If fNameAndPath = False Then Exit Sub
    
    'MsgBox fNameAndPath
    
    fName = Right(fNameAndPath, Len(fNameAndPath) - InStrRev(fNameAndPath, "\"))
    
    fName = Left(fName, InStr(fName, ".") - 1)
    
    'MsgBox fName
    
    numberOfLines = CInt(InputBox("Enter the number of lines per file", "Enter number of lines", "100"))
    
    Open fNameAndPath For Input As #1    ' Open file.
    Do While Not EOF(1)    ' Loop until end of file.
        outputFile = fName & "_" & fileCount & ".txt"
        'MsgBox outputFile
        Open outputFile For Output Shared As #2
        Do While (myCount < numberOfLines And Not EOF(1))
            Line Input #1, TextLine    ' Read line into variable.
            Print #2, TextLine    ' write variable to output file.
            myCount = myCount + 1
        Loop
        Close #2
        
        fileCount = fileCount + 1
        myCount = 0
    Loop
    Close #1    ' Close file.

End Sub