The Department for Transport does not keep a record of how many train companies have (a) become bankrupt, (b) gone into liquidation, or (c) ceased trading. This information may be sourced through Companies House. The decision to cease trading is a commercial decision made by train companies.
With regard to the number of train companies which have started up in each of the last 10 years, this is a matter for the independent Office of Rail Regulation who can be contacted at:
Office of Rail Regulation
The Office of Rail Regulation provides all train operators on the United Kingdom rail network with operating licences.