That presumably depends on how many IETs are currently diagrammed for non-IC duties. Obviously it's quite a few, but I doubt it's getting on for 20, which is presumably roughly how many 175s will be available for service once the HSTs have been replaced.
I think it nearer 7 or 8 5car IETs will be released (3 on Cardiff-Exeter leg, couple doing full Cardiff-Plymouth, 1 or 2 used on Bristol-Worcester locals, 1 on Cornwall local covering lack of DMUs).
Exeter-Cornwall still uses Castle HSTs, 3 units (or is it 4) each day. Although a few spare sets still exist, even if not in service
So around 70 class 175 vehicles, works out about 12- 16 vehicles releasing HSTs for scrap, around 35 vehicles releasing IETs, and roughly 15-22 for Devon and Cornwall services like Oakhampton (depending on how count spares required)
In practice there are handful of 158 and 16x units (around 10-15 vehicles) semi-permanently out of service, because of lack of spare parts, so assuming these are parted out to keep rest of fleet going then Bristol will only gain a net of about dozen vehicles (to cover enhanced MetroWest and the stock shortages on Portsmouth-Cardiff).
So with GWR growing in the west, by next summer, even if all the 175s are in service, unless there is a massive investment in spare parts for 165x fleet, probably still be capacity problems, and having to use IETs on local services.