Yep, when raising the team bonuses there's only one criteria you need to consider - The amount of players participating in the session (of each role-type ofc.)
Attack - 4 Attacking players
Defense - 4 Defensive players
Possession - 4 Midfield players
Condition - A minimum of 8 players in the session
There is a minimum of players you have to employ but there is no cap. And the more you have the more will contribute to the bonus.
Personally in my case it takes me 3 boosted training sessions to go from 8% to 10%. Some days it takes 4 boosted sessions to do the same. But keep in mind that the effectiveness of the training session can vary and you can gain more or less progress for each of the covered bonuses. Of course that means that the condition loss also varies as observable in the training reports.
We can all agree that a lot more training progress is required to fill up the 8% to 10% margin than for the 0% to 8%. I'm reaching 8% (and in rare cases a little bit after that) by doing all of the 4 boosted sessions. But that's why if you want your bonuses to be high or even capped you should try not neglecting them. I personally try to keep my self at 6% or more depending on the upcoming matches, this way I can "reliably" cap my bonuses at the day of a specific match I expect to be hard.
I've had several occasions when the progress bar of my Def Bonus would fill up (almost it seems) to 10% but the bonus would stay at 8%, because the session couldn't find it in it's hear of hearts to sprinkle those last few specks of sand and fill up the damnable bar to the maximum! ._. /rant over
Now, what the drills contribute towards is:
1) Condition loss (Again as you've definitely noticed, the end loss can sightly vary)
2) Player Attribute experience
3) Training Level experience
This is written on the card of each drill.
I've added some screenshots so you can weep together with me about those 8% never making it to 10%.
(And they had so much potential to become 10s too...)