Well, I think that line derives from the fact that the fifth Doctor might expect to regenerate in any normal circumstance, but he isn't sure if spectrox poisoning is something he can recover from - but otherwise, he thinks he's got eight more regenerations left to him.
Exactly. It's a mistake to think that regeneration is always
a guaranteed out from death. It won't work if the damage is too great, so there's usually a risk that it won't kick in -- which is why the Doctor isn't just blase and casual about it when it happens. We saw this with the third regeneration in "Planet of the Spiders" -- the Doctor almost didn't regenerate, but needed K'anpo's help to jumpstart the process. And while he did regenerate after "Logopolis," we saw that there was a risk of the regeneration failing and the Doctor not surviving -- or at least regenerating again right after, and who knew if the next one would be stable? (See also "The Christmas Invasion" for a touch-and-go post-regeneration phase.) And of course we saw in "The Impossible Astronaut" that if the Doctor got fatally injured during
regeneration, it would halt the process and leave him dead.
With something like the injuries the Doctor sustained in "Logopolis" or the McGann movie, where the cause of the damage is over with by the time regeneration starts, it's just a matter of healing the damage that's been done. But in "Androzani," with the spectrox toxin still coursing through his body and doing lethal damage, the Doctor couldn't be sure the regeneration process would be able to cancel out that damage, rather than vice-versa. And perhaps the iffiness of that regeneration was why the Sixth Doctor was so erratic at first.