The British censuses give an accurate assessment of the number of professional photographers in Victorian Britain. The 1881 England headcount lists 4,722 results for ‘photographer’.
It is important to remember that early photographers were sometimes listed under different occupational headings. In the 1851 census, for example, you will find records for ‘daguerreotype artists’ (from their use of the daguerreotype process).
Also, not everyone who worked in a photographic studio was a photographer. You should also consider occupations such as ‘retoucher’ (the person who worked to ‘improve’ glass negatives) or ‘colourist’ (the one, usually a woman, who hand-coloured photographs).
For England and Wales, you can access census records at the National Archives in Kew and online at various commercial websites. Regional census returns may be found at your local reference library.
For Scotland and Ireland, see the General Register Office in Edinburgh (also available online, for a fee, at www.scotlandspeople.gov.uk) and the National Archives of Ireland www.nationalarchives.ie.