I've done some more investigation and it seems that how your audio is routed can make a BIG difference to your ASIO meter, and to how many plugins you are able to run.
I ran three tests, each one using 24 instances of Waves Kramer Master Tape (KMT).
#1. ASIO meter at 15% , Win CPU at 15%
24 x stereo tracks, each with 1 KMT, no group tracks.
#2. ASIO meter at 45% , Win CPU at 11.5%
3 x Stereo tracks, each with 8 KMT's, no group tracks
#3. ASIO meter at 100% , Win CPU at 11%
1 Stereo track, with 8 KMT's, routed to two groups in series, each with 8 KMT's. i.e. a continuous chain of 24 Kramer instances.
So... it appears that running long chains of plugins (i.e. like test #3) is a LOT more ASIO intensive than many short chains of plugins. It is also clear from the Windows Resource Meter that long chains can not be spread between CPU cores as effectively as multiple short chains (Test 1 showed a perfectly even spread between all cores. Test 3 was very uneven with some cores seeing virtually no load at all).