You may want to grow your sclerotia in erlenmeyer flasks of 1 or 2 liters on imbibed sterilized grains. This can give much higher rates of sclerotia than in petri plate. Grain such as wheat, barley sorghum and corn can probably do the job. Make sure you sterile in the autoclave for 2 to 3 successive days and let the Rhizoctonia colonize the grain and shake the stoppered growing units after you have good surface growth every week to get them thoroughly colonized.  For field inoculation this is what most breeder pathologist do to increase inoculum at a large scale for field inoculations. 

I am not a scientist, so this may be too basic for your needs, but perhaps the attached may help: Inoculum Increase Of Rhizoctonia Solani Ag-2-2 Sclerotia Formation On Sugarbeet Roots By Cheryl A. Engelkes and Carol E. Windels 

It discusses the production of sclerotia on sugarbeet roots  in field experiments and contains some procedures and factors which affected the number and timing of sclerotia formation , including varieties with tolerance to Rhizoctonia root rot, favorable root surfaces for sclerotia formation, and source of cultures.

INOCULUM INCREASE OF RHIZOCTONIA SOLANI AG-2-2 BY

 SCLEROTIA FORMATION ON SUGARBEET ROOTS

Cheryl A. Engelkes and Carol E. Windels

Former Graduate Student and Plant Pathologist, respectively

Northwest Experiment Station

University of Minnesota, Crookston

Attached and available here: http://www.sbreb.org/research/plant/plant91/91p196.htm

Cheers,

Leo

Deer Aqleem. 

i suggest you to grow the fungus on sterilized Wheat or Barley straw or Rice hay

Best Regard

Hi Alqeem

You can read this paper "Survival of Sclerotia of Rhizoctonia solani AG3PT and Effect of Soil-Borne Inoculum Density on Disease Development on Potato"

http://onlinelibrary.wiley.com/doi/10.1111/jph.12052/abstract

Dear colleague,

I would like to ask first about the purpose for getting high count of R. solani sclerotia?

Do you need it for pot experiment? If so, I do not prefer to increase inoculum using biotic   carrier. Simply if you want to get high amount of sclerotia you can grow the fungus in large Petri dishes ( 15 cm in diameter for at least 15 dayes then you can find high number of sclertia.

Yes, I want to bury sclerotia in pots and used as baiting test.

See the attached file and do the same search steps