Have you had your soil tested by a laboratory? It would be helpful before planting to get the pH or acidity level and see the level of nutrients in the soil. Rhododendrons need well-draining fertile soil but also soil that leans more acidic, so if necessary you would want to add elemental sulfur to lower the pH. 

You can look for a lab to use and find more information on how to submit a soil sample on our webpage: Soil Testing and Soil Testing Labs

We also have a page soil health and drainage that you can refer to for helping to improve clay soils. Clay soil has the right nutrients, but the drainage that can be an issue for some plants.  You can perform a 'perc test' to see how quickly your soil is draining in the location you want to plant in, and adding the organic matter will help improve that. 

There isn't really a defined quantity to add, but if you top your planting area with compost and wood mulch this winter, it will help to improve the soil by spring. 

If you would like help reading your soil test results, you can attach them to a response. 

Let us know if you have further questions. 

Emily

You should be fine with using just good-quality topsoil (which is not a regulated material in Maryland, but you can inspect it in person and ask the supplier for their soil test results before buying). You could amend that with some purchased or homemade compost if you prefer, but you don't want to plant in pure compost. As for area size to amend, as Emily noted, there is no set volume of soil, but aim for a wide amended area rather than a deep one. Most tree and shrub roots remain in the top foot or two of soil, and rhododendrons in particular (and their azalea relatives) naturally have fairly shallow roots. Like all shrubs, their roots will primarily grow outwards, away from the center of the plant, and also not far under the surface. If you imagine an uprooted mature plant, the root zone would look more like a thick pancake than a more spherical spread of roots belowground (like a reflection of the canopy). If in doubt about drainage even after amending/replacing soil, plant a bit high, with the root flare (where roots branch off of the trunk base) a couple inches above-grade, as long as the soil mound won't erode. (Mulch will help to keep it in place, but don't pile mulch against the main stems.)

Any soil or compost you add to the site should be thoroughly mixed with any existing soil that roots may encounter, since you don't want the differences in drainage and moisture retention between those materials to stress roots.

Miri