"Are there nineteen or twenty of them?"
Translation:Je jich devatenáct nebo dvacet?
1) Nominative (and accusative) phrases with numbers 5+ are treated as neuter singular for verb agreement (I know it's weird, but that's the way how things work.) So you would need to use je instead of jsou.
2) Overall, your sentence sounds super weird and unnatural. :(
I'd stick to Je jich devatenáct nebo dvacet? or Je jich tam devatenáct nebo dvacet? (use tam to emphasize the location 'there')