It sounds like the lecturer is saying
how many of these are actually calculatabe from others ...
I didn't find this word in cambridge dictionary
Is this a spoken mistake? which should be calculable
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Assuming ‘calculatable’ was intended: this will be accepted as valid but non-ideal spoken English by most hearers.
‘Calculable’ would be better, but there is a reason why your lecturer used the alternative.
There is a tendency to use -able as a suffix to completely unmodified verbs in spoken English.
Language is a statistical entity. Informal language is even more probabilistic and less rule-bound.
One might expect a lecture to be more formal in nature and not less, but that is not necessarily so.
When delivered from prepared notes or a long hand script a lecture may be quite formal, true.
And yet extemporaneous speech—though it may adhere to a certain high register—is more taxing for the speaker to generate continuously while still following some set of grammatical (or other) rules exactly.
There is little conscious deliberation possible in a monologue (as opposed to a conversation of multiple parties) and the thoughts will be expressed in common patterns that are freely available to the speaker.