What weight is this stock in 12 x 18??

If you're looking for the shipping weight of the OL977WX, please reach out to Customer Support at 888-575-2235 for those details as it would be based on the quantity of sheets. If you need the paper weight or thickness, you can review our material specifications below. These are found on the material page as well, https://www.onlinelabels.com/material_white_matte_labels.htm . Facestock Weight: 52# ± 10% Facestock Thickness: 3.9 mil ± 10% Liner Weight: 45# ± 10% Liner Thickness: 3.3 mil ± 10% Total Thickness: 8.0 mil ± 10%

Is there only 1 vertical slit on liner? or multiple?

The OL977 has 1 vertical back slit for easy peeling. If you need a 12" x 18" with multiple back slits, please visit the link below for our OL978 which has 2 horizontal back slits. https://www.onlinelabels.com/products/ol978