If the answer is "a lot" then last night was successful for Trump.
I'm still not really sure where you get the assumption it energized his base relative to Biden. Biden literally set an hourly donation record during the debate which is a pretty strong indicator that what his supporters saw was
more motivating to get out and vote not less. And then there are things like this:<blockquote class="twitter-tweet" data-conversation="none"><p lang="en" dir="ltr">This is kinda interesting, aside from just who "won" debate<br><br>Slightly more watchers say debate made them think better of Biden (38%) than worse of him (32%): nets to +6<br><br>When it comes to Trump, more watchers say it made them think worse (42%) than better of him (24%): nets to -18 <a href="https://t.co/0tVuOg1h3K">pic.twitter.com/0tVuOg1h3K</a></p>— Kabir K. (@kabir_here) <a href="https://twitter.com/kabir_here/status/1311148351811915778?ref_src=twsrc%5Etfw">September 30, 2020</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
Biden +14 versus Trump in people leaving with a positive impression, and +6 favorability. Trump -14 relative Biden, and -18 favorability. This would suggest if people watched that and now want to stay home they're likely on the right not left.
To your larger point though, only 6% of people who tuned in said they were doing so to help decide who to vote for.
This tracks with the ~7% "undecided" voters we've been seeing in polls for awhile. So you're correct that there is a relatively low vote share of undecideds.