(1) The king had a great amount of interest in observing the universe and stars (OK)
(2) The king had interest in observing a great number of the universe and stars (???)
Is (2) possible in terms of grammar and context?
No. The first sentence isn't ideal, but is okay. ' but I've seen it expressed as you have.
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No. The first sentence isn't ideal, but is okay. We would, more naturally, say, '...had a great interest in...' but I've seen it expressed as you have.
The second sentence uses the 'great' quantity not with 'interest', but with 'universe and stars'. Given that there is only one universe (hence the prefix 'uni-'), there cannot be a 'great number' of them.