Extract value from HTML Source

If you go and work with .responseText using Split() doing text manipulation you might as well use a regular expression without setting it's Global parameter:

Public Function UnitPerBox(searchTerm As String) As String
Static request As Object
If request Is Nothing Then Set request = CreateObject("msxml2.xmlhttp")

Dim RegEx As Object
Set RegEx = CreateObject("VBScript.RegExp")
RegEx.Pattern = "\d+(?:\.\d+)?"

With request
    .Open "GET", "https://larsonjuhl.co.uk/mouldings/larson-juhl-essentials/arq-essentials-moulding-" & searchTerm, False
    .send
    UnitPerBox = RegEx.Execute(Split(.responsetext, "Units per pack</td>")(1))(0)
End With

End Function

Neater (IMO) however is to avoid text manipulation on the .responseText alltogether and work through the HTML document, retrieve the appropriate data straigt from the HTML-table by element-ID and table indexes:

Public Function UnitPerBox(searchTerm As String) As String
Static request As Object
If request Is Nothing Then Set request = CreateObject("msxml2.xmlhttp")
Dim htmlResponse As Object: Set htmlResponse = CreateObject("htmlfile")

With request
    .Open "GET", "https://larsonjuhl.co.uk/mouldings/larson-juhl-essentials/arq-essentials-moulding-" & searchTerm, False
    .send
    htmlResponse.body.innerHTML = .responseText
    UnitPerBox = htmlResponse.body.document.getElementById("specifications").getElementsByTagName("tr")(10).getElementsByTagName("td")(1).innerText
End With

End Function

Note that the table is 0-indexed meaning we are actually retrieving our value from the 11th row, second column. In case you are not sure that the tablecontent is always found on the same indexes, you could also just loop the child nodes:

Public Function UnitPerBox(searchTerm As String) As String
Static request As Object
If request Is Nothing Then Set request = CreateObject("msxml2.xmlhttp")
Dim htmlResponse As Object: Set htmlResponse = CreateObject("htmlfile")
Dim Rws As Object

With request
    .Open "GET", "https://larsonjuhl.co.uk/mouldings/larson-juhl-essentials/arq-essentials-moulding-" & searchTerm, False
    .send
    htmlResponse.body.innerHTML = .responseText
    Set Rws = htmlResponse.body.document.getElementById("specifications").getElementsByTagName("tr")
    For Each Rw In Rws
        If Rw.getElementsByTagName("td")(0).InnerText = "Units per pack" Then
            UnitPerBox = Rw.getElementsByTagName("td")(1).InnerText
            Exit For
        End If
    Next
End With

End Function

Where I personally would prefer to use HTML document over text manipulation, all above options work to retrieve your value =)