Do we need to supply company RegistrationNumber in specific format?
Any special characters not accepted/allowed by API?Any max length limit with the number?
Yes, full 8 characters, zero padded, any letters in uppercase.
As Mark says.
For the full details of what company registration numbers can be, see e.g. p.7-8 of the company URI documentation at https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/426891/uniformResourceIdentifiersCustomerGuide.pdf
The list in the published link DOES NOT INCLUDE PC (Protected Cell Company) or SG (Scottish Partnership) which were added 07/04/17 (V1.11 d). These are the latest changes I have but I have not looked for any others. There have been several updates to codes since the URI Customer Guide version on the link though these may not affect company number prefixes.