All I have to do is field 8 players from my squad at the start of the match. Any changes during the match won't affect the sponsor?

It's all about getting players from your academy to play together.
It's not necessary for them to be lined up at the start, but if you start with fewer than 8 and make substitutions during the match to get there, that works too.
(Just go to the sponsor page to see how the bar is progressing, it updates during the match)

As I said earlier, there is a minimum time (I can't confirm what it is). I've sometimes made changes and seen the bar rise... Be happy .... Making changes again, to avoid injury ... To see the bar drop again at the end of the match ....
sniff ... :'(

[Warning!
If, before the end of the sponsorship period, one of the players is no longer a player: if you convert a retiring player into a staff member. In that case, the matches that were counted with him will no longer be counted = the sponsor's progress bar will drop again.