Unfortunately you haven't given an indication as to where you are from, so I can only speak for my airline in the UK.
A crew member will complete their initial cabin crew training and will have to then complete 8 familiarisation sectors, operating as an addititional crew member (i.e. 4 in the cabin instead of 3 on a 733)
Once the crew member has completed their 8 familiarisation sectors, they can then operate as a fully fledged crew member (i.e. there would be 3 in the cabin only).
BUT for the next 60 sectors that that new crew member operates, they must be rostered with fully experienced crew. This prevents you having the problem of just 1 experienced purser and 2 very junior crew in the cabin who could potentially be a bit green in an emergency
