It sounds like the lecturer is saying
how many of these are actually calculatable 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.
The suffix -able is a productive suffix in Modern English, it can be appended to an enormous number of words without altering the the root word. For instance, envisageable sounds very natural even though it's not present in reliable online dictionaries.
In some cases, it does alter the root word as in calculable, but generally, it doesn't. The lecturer said calculatable probably by analogy with other words whose pronunciations don't get altered.