Often, we have 2.6 year olds that are either siblings or friends of previous/current children at the preschool.
Most 2.6 year olds do around 2 sessions per week, simply because it costs the parents too much if they are not funded. Having said that, occasionally a parent will pay for their child to have more sessions, as it is their main childcare source.
Staffing is quite flexible, I think. Staff are contracted for a minimum set of sessions, then offered overtime during busier times. They receive all of their annual leave as a lump sum during the summer holidays, so are heavily discouraged from taking leave during term times, although at discretion of the manager they can if essential.
You know how many children you are allowed due to OFSTED restrictions. You can convert that into number of sessions you can offer (ie. max 26 children per session, 16.5 hours open per week, so 429 hours per week total to offer.) You also know that most 3 year olds will take up 15 hours per week. So you know that you can have a maximum of 28 children on your books if all of them take hours neatly to use up all your availability.
Although your end point for the children is fixed to once per year, your start point will vary.
January born - July start, April funded
February born - September start, April funded
March born - September start, April funded
April born - October start, September funded
May born - November start, September funded
June born - December start, September funded
July born - January start, September funded
August born - February start, September funded
September born - March start, January funded
October born - April start, January funded
November born - may start, January funded
December born - June start, January funded
So you will find that you start Autumn term with less children, then build towards Summer, then the cycle starts again.