Hi Debbie, I undersand your distress at seeing your Mum bruised, but with the best will in the world and all the staff you could ever want, it is impossible to be sure that things will never happen. Bruises happen for all sorts of reasons, the elderly are more prone to them due to weaker cell walls, some medications can cause a weakness, a knock on a wall, a chair, a table and there is another mark. My own late Mum could bleed for England at the drop of a hat. Having said all that, there are times when bruises can be caused by abuse, and sadly Dementia can cause behaviour changes causing physical attacks.
I googled " ratios of care staff to Care Home Residents" and found this :
http://www.rqia.org.uk/cms_resources/Staffing Guidelines for Residential Care Homes.pdf
Now this relates to 2009 and Residential care home settings, but there may be more up to date ones known to other members.
Good Luck .... Maureen.