How can I cover these wooden numbers when I paint?

Marine spar varnish

You can buy paint sealant to protect painted objects such as your numbers from the weather. You can find it at most places that sell paint. I use the spray ones, as they are much easier to put on and seal all around the wood, even on the sides and where they are attached to the concrete.

You could try laying on them.

Use a spray sealer for wood. You can find it at any hardware store.

Hi chris. Howzabout painting over them as it could serve as an undercoat, then paint them your choice ov colour.

Helmsman Marine Spar Urethane. Used on boats.

only paint .. concrete doesn't have to be protected in a different way

Spar varnish will definitely work, after all it is made for boats.

You can cover them in painters tape which is like blue masking tape easily found in the paint section of your hardware store. Or if I misread your question, you can cover the concrete in newspaper using the painters tape right up to the numbers. An exacto knife to uncover the numbers only so when you peel off, you won't take any of your new paint layer with the tape.