In the US there is no limit (see 14 CFR Part 382). In the EU (EC)965/2012 sets the ratio to 1 to 1 (one able body per one SCP). However, you should note that SCPs include children under the age of 12 (accompanied or not), pregnant women, deportees and prisoners as well as passengers with disabilities.
Some low cost carriers impose collars on the number of WCHC per flight (two or four) which is unrelated to safety, but rather to contain turnarounds within 25/28 minutes.
Hope this helps.