number of optical isomers of a cyclic molecule
1-Bromo-3-chloro-5-fluoro-7-iodoadamantane would be an interesting example of central tetrahedral chirality without the actual (physical) center. Exchanging halogen atoms will result in only two stereoisomers.

Due to its rigidity, its stereochemistry is related to tetrasubstituted tetrahedrane (tricyclo[1.1.0.02,4]butane; or rather its imaginary geometrical model, because it would be very strained and unstable), whose stereochemistry is related to bromo(chloro)fluoro(iodo)methane.





To your additional question: It depends how (far) you'd like to go with removal of the rotation restriction, anyway the resulting compounds will already not be original 1,2-dichlorocyclohexane.



I mean yes, (1R,2R)-1,2-dichlorocyclohexane and (1R,2S)-1,2-dichlorocyclohexane can probably be considered geometrical isomers (in addition to optical), they can even be named trans-1,2-dichlorocyclohexane (which has two enantiomers, the other one being (1S,2S)) and cis-1,2-dichlorocyclohexane; but this isomerism is not caused by rotation restriction, but by different arrangements on chiral tetrahedral carbons.

But note that geometrical isomerism is usually used in relation with double bond cis/trans isomerism; and together with optical isomerism are both deprecated terms.