[QUOTE=Garfield Cat;17890599]If a rider does some steady efforts intervals, say 8 of them. The first interval will feel much different than the 4th. and then the 8th is way different and most exhausting as the desire to maintain that intensity starts to wane.

Is there an optimal number of intervals here?[/QUOTE]

Of course... The optimal number is the number given by whoever you ask -- and if you ask 5 people you will get 5 different answers. And that includes not only number of intervals but also the intensity and duration of each interval -- as well as the rest periods between them. And, it even includes how you measure/determine the intensity...

Exercise physiology is still a very crude science (if you can even call it a science)... And, HIIT training being a relatively new addition to the field, it is perhaps less of a science than most other forms...

That doesn't mean it is not beneficial. But, when you start getting down to the details and nuts and bolts, you end up with a lot of educated guesses... But [heavy sigh] that's the best we have to go on right now...

Personally: my exercise physiologist suggested: 3 or 4 sets of: 4 minute high intensity at 95+% MaxHR followed by 4 minute rest at 85% MaxHR. But, if you change either the duration or the intensity or the rest periods, then you should also adjust the quantity of intervals (and vice-versa)...
... Some people prefer 10 sets of 30 second intervals of high perceived effort.

Perhaps the best general advice is: "Do it till you see Jesus!"