sounds like too many serves to me, unless you have worked up to it.
I'm thinking 20-50 serves, 2-3 times during a practice.
Try some serving into the back fence where there is little focus on target and
much focus on swing. See what you think.
We usually do 5-10 of these, then do much better when serving into the court.
If I feel they start to worry too much about the target again....we just
shift back to the back fence again.
Swing almost always looks way better and more powerful into the back fence,
then often loses some luster when going over the net.
We go back and forth till we can keep the aggression into the back fence in
force when we are serving over the net.