It's not how many hours that matters for a first solo, the basic questions are:-
"Am I satisfied the student has attained the level of airmanship and skill necessary for a first solo?"
" Are the conditions suitable?"
"Has the student completed all the necessary theory work and examinations for a first solo?"
"Does my student pilot know within themselves they are ready for solo?"
How many hours they have done is only of interest as a gauge of their overall progress, and that may vary according to how frequently they have been able to train, how bad the weather has been over their total training period, serviceabliilty of training aircraft etc.
Students don't delete their hours under instruction after they start logging hours solo. Every hour in the air is an hour's experience. This applies equally for those on the long, hard road to heavy jet command as it does to the weekend recreational pilot flying an ultralight.