It depends on the specifics. You can now usually find out the pre-schools particular registration via the Ofsted website.
Lets say the group meets in one large hall. Typically they may be registered to care for 26 children, of whom 6 can be aged 2 to 3 years.
With 6 children under 3, they need 2 members of staff, as 2 to 3's have a ratio of 1:4
For the children aged 3 to 5, the ratio is 1:8, so taking our example, that's 20 children : 3 adults.
So given the example, I would expect them to have 5 adults, if having 6 under 3's and 20 3-5's.
Another possibility is that they could do a splt ratio, so with 26 children, 6 of them under 3, could be done by doubling the 6 children, to 12 adding to the remainder to give 32, then doing 1:8 on that, which results in 4 adults as a minimum.
If they operate from more than one room, then things change again, as there are requirements for there to be at least 2 adults per group (which I interpret as being per room - as you can't see through walls!).
1/2 of the staff need to be qualified. 1/2 can be unqualified.
For a pre-school (sessional group) you can find the relevant National Standards here Staffing Ratios are in section 2.7
Hope that helps.