It will depend upon your transformation efficiency. We just try to use large plates to minimize the total number of plates, so standard large petri dishes (somewhere between 140 and 160 mm, I think) will work. You could even use a much larger media tray like the Qtray (see link) - we use them with the dividers but those are optional.

http://www.moleculardevices.com/reagents-supplies/bioassay-trays/bioassay-qtrays

For large screenings I've used 24.5 cm x 24.5 cm square dishes (e.g. from Corning). This makes 500 cm2 of culturing surface and can be gas-sterilized (do not autoclave them!!!) if re-use is required.

Hope that ths helps

Estanis 

Have you determined the transformation efficiency?

The following link explains an approach and I think it is better to find that first before trying to find number of petri dishes you need given the area of the petri dish you use. In fact you can follow Caufield and Navarro suggestions.

https://books.google.co.uk/books?id=-kf2FXUp1rwC&pg=PA206&lpg=PA206&dq=To+determine+the+transformation+efficiency+of+the+Y190+carrying+the+bait-fusion&source=bl&ots=92CggbqE-u&sig=qmsrWfIkobgDguqJChziWkmu6R4&hl=en&sa=X&ei=p_ErVfTgD5bxarvNgGA&ved=0CCIQ6AEwAA#v=onepage&q=To%20determine%20the%20transformation%20efficiency%20of%20the%20Y190%20carrying%20the%20bait-fusion&f=false