Should you have any enquiries 個enquiries 應該係單數定眾數?

it is more correct to say "should you have any queries, please do not hesitate to contact myself....."

when you use "enquiry or enquiries" this is made more in the past tense of english, i.e. implying that something happened in the past.

therefore in your sentence you want to use the future tense of english, because it is something that may or may not happen in the future.

I would say they both correct.

應該是enquiry，普遍較常用。