Tee Times are set for each group in League Setup -> Step 4 Tee Times. The initial setup wil be your "default" tee times. Since most leagues play the same times each week, this is a one time setup and applies to each round without any other changes on your end.
If you play one or more rounds at a different time from your "default", you can click on the round number on the right side, then change the times for that specific round. The round will be highlighted in pink to show you it's using "custom" tee times.
If you have added players or teams to your league this year, you need to set your times for the additional group(s) that were added to your league.
Golf League Tracker calculates the number of tee times for your players by dividing your player count by 4 (or, if you're playing a scramble league, dividing your team count by 2). If you have 24 players, that's 6 groups, hence 6 tee times. However, some leagues have a larger block of time reserved than this, and you can tell Golf League Tracker the number of tee times you want with Setting #248 "Minimum Number of Tee Times". If you set this to 10, but only have 24 players (6 groups), this will give you 10 groups instead, and you can set the time for each of those groups.
You can never have fewer tee times than what Golf League Tracker calculates based on your player count. If you have only a portion of your players playing, that's ok, you'll just leave those groups blank when setting your schedule, but be sure to set your tee times for each group, just in case you use them in the future.
You can turn off tee times from being displayed in the schedule or on scorecards with Setting #138 "Show Tee Times"