Looking at today, you've got a 1007 RDG-BSK 1035, Next working 1053 BSK-1119 RDG, next working would be 1133 RDG-BSK (if they're not interworked with a Newbury or similar). So I think it's 3 units in the cycle for the approximately half-hourly service. [1007, 1033, 1107 departures from Reading before the 1133 is formed off the ex-1007.]
There are also other reasons to electrify this area:
1. The 2 most difficult things are basically done - power and signalling.
2. It puts EMUs on stopping services, where their acceleration can be best used.
3. It replaces DMUs on the busy Southcote jn - Reading section, where EMUs will be able to get through slightly quicker.
4. Allows a 4-car EMU to replace a 2/3-car turbo.
5. Allows greater flexibility in fleet allocation with Reading-Newbury and Reading-Basingstoke both fully wired.
6. It increases the utility of potential XC and freight bi-modes, aiding the case for procurement.