More or less one additional unit diagram is needed for the Bristol service, since it's a two hourly service and it takes an hour each way from Cardiff. The long turnround in Bristol (1h20m on most services) is basically the same as what the West Wales services currently have at Cardiff.
However because the long turnround is in Bristol rather than Cardiff, the marginal increase in the number of crew diagrams is likely higher than the increase in the number of unit diagrams.
For example whilst a conductor might be able to take a break in Bristol, it's ultimately still time that counts towards the length of their diagram and thus limits the number of round trips any one conductor can perform in a day. The drivers generally won't be able to take a break because the units will be shunting via Bristol West Jn during the turnround.
One 'out and back' run from Cardiff is around 3h20m; multiply that by 9 daily return trips and you have roughly 32 hours of work a day to cover. I don't know TfW's diagramming parameters but that's going to take at least 4 driver + 4 conductor diagrams when you account for breaks, even if it's works out 100% efficiently which is never the case. Probably more like 5 driver diagrams when you account for the fact they can't take breaks in Bristol.
The ratio of diagrams to headcount varies depending on the TOC, but with TfW having a 4 day working week and now having Sundays inside the working week, you're looking at a bare minimum of 2 staff per diagram. Again the average is likely to be higher at 2.5 or 3 when you account for covering annual leave, sick leave, training etc.
So very roughly you are talking 12 drivers and 10 conductors. Based on published salaries and pension contributions rates, their direct employment costs work out at around £1.8m a year. Divide that by the roughly 3000 services a year that TfW will run and you get to a figure of around £620 per round trip, more than twice the estimate above.
Based on those figures it would take at least 35 passengers paying the full adult fare to cover the staffing costs. Whilst you may well have a higher loading than that on some trains, how many will be on Railcard discounted, season or child tickets? There's also sales commission of roughly 5-10% depending on the retailing channel to consider.
When we consider that staffing costs are only around 28% of operating costs as cited, these services are going to need to be nearly fully occupied on every service to break even....