Thank you for your answer!

It will be less than a few thousand records, we are selling 250 lots, and
there are always a few canceled ones, but the total records will still not
come close to a thousand; so DCount is probably the best way to go. I do
however have a question as I might not have been totally clear when I posted
my initial question. I only want the messagebox to pop up if the user will
asign a lot number that has already been used, not everytime assigning a lot
number. If the lot number has been used earlier on AND the client status
combobox sais "Canceled" I want the message box to pop up.

I am using a combobox listing the numbers 1-250 to store the lot number. I
tried changing the code to match my combobox (Combo215) instead of the
standard textbox:

Sub Combo215_AfterUpdate()

'txtLotNumber is replaced with Combo215
'For reference: My table is called RegisteredClientInformation
'I also replaced LotNumber with Lot, just as it is called in my table

Dim x As Variant

x = DCount("Lot", "Registered Client Information", "Lot = " & Me.Combo215)

If x >= 1 Then
    If MsgBox("Lot number has been used previously, but was canceled. Save
new client?", vbYesNo, "Found") = vbYes Then
        DoCmd.RunCommand acCmdSaveRecord
    End If
End If
End Sub

I keep getting an error message:
'"Runtime error 3464":
Data type mismatch in criteria expression.'
The data type of my Lot combobox is text. Do you have any suggestions?

Thank you for your time and help!