There is an article on here about caffeine content of energy drinks by Johnson et al. Gov.uk, FDA and European Food Authority may also be good places to look.
As far as I know the concentration is not limited here in Finland, but if content is over 150mg/l the package needs to have a warning of high caffeine content. Some shops have started to apply age limits (15+) on energy drinks, but this is not yet govern by any law.
I suggest reading the efsa report from 2013 https://www.efsa.europa.eu/en/press/news/130306
Not really. Various manufacturers of energy drinks get around any "toxic doses" (usually 400mg and >) of caffeine by giving doses per "serving" (8 ox) or per 1 oz. Usually the standard dose is 200mg +/- 10%.