The designer is free to place as many question blocks as he can decently fit into the desired size of paper. Technically, there is no limitation to have any number of blocks or questions on a single side of a sheet of paper. However Duplex (two sided) OMR Sheets can also be designed and read by the software.
Some sheets like Application forms and answer sheets with large number of questions which cannot be fitted onto one sheet of paper on one side so the OMR layout is made such that the fields are placed on both sides of the page. Such sheets are called duplex sheets. Generally fields related to one type of information are clubbed on one side of the Duplex sheet page.
For example, if a Test Answer Sheet is collecting Students Name, Mobile Number, Roll Number, Choice of Course and subject plus 50 Questions with 4 options, then this can be conveniently managed using simplex or single sided OMR Sheet. But suppose if the number of questions is 300 or say 200 with 5 options with all the other fields, then it would not be possible to fit it into one page on one side. For this type of case there is a need for making a duplex OMR, and for convenience sake, the Name, Mobile No., Choice of Course or subject and Roll No. fields can be kept on one side of the page while the other side may purely contain 200 questions. This helps in creating designs that are not too crammed up and are technically correct as per the norms of OMR designing.
Duplex OMRs are mostly used in major examinations and private scholarship events where the purpose of the examination body is to collect information of the candidate as he also solves the question paper. So the number of fields to be incorporated onto OMR design increase makes it necessary to make use of duplex sheet layout.
Duplex scanners which can scan simultaneously on both sides of the page are required for the purpose. Normal scanner that we use in our offices do not have the facility to scan duplex OMR sheets. Read more about scanning of duplex sheets. Choice of scanners for Duplex Sheets.