Where can I find the total number of death penalties sought in the state since @1973?

Nowhere. That information is not reported to any centralized agency, so it goes uncollected.

The Office of Court Administration (www.courts.state.tx.us) keeps some general statistics on capital murders indictments and death sentences, but it's county-by-county, year-by-year. If you have a lot of time on your hands, you could be the first person to arrive at that figure ...

And I think all the OCA keeps track of is the number of capital murder indictments (potential death penalties) and number of death penalties imposed. The cases in which the prosecution actually seeks a death penalty, but it is denied by the jury, are not reported in that fashion (as far as I know). So you would have to have a lot of time and energy to spare to answer this question. And why bother, the end statistic will essentially be meaningless because each of these cases is essentially unique.