I have a area of my garden which has concrete under it so I only have a few inches of soil to plant plants. I plan to buy some compost to make it deeper.
I wondering which plants are ok with thinner soil.,
Sedums and other succulents do well on thin soil. Bearded irises may work as well.
Are you planning on building a raised bed with sides on this patch, to keep the soil in place? With concrete underneath you will need to consider how to provide drainage so the plants don't get waterlogged.
Rather than buy compost for this patch, I would buy equal quantities of topsoil and bags of farmyard manure - both available in bags from garden centres - this will make much better growing conditions long term.
Do you want to grow veg or flowers?
Is the area in sun or shade?
If you can't take the concrete out go for raised beds. You'll never have success growing anything on top of a concrete slab. Water has nowhere to go when wet and when its dry the compost will just dry out.
how deep does the soil need to be.
its a bright area.
Here is a pic - its the area near the blue light.
You would do better to scrape off the existing soil from the concrete and stand another large 60cm or larger pot on it, You can use gravel to hide the remaining concrete. A big pot can hold anything from a shrub or a rose to a columnar fruit tree depending on how much sun it gets.