A VAT number is a unique identification that HMRC provides when a business registers for VAT. VAT numbers in the UK are 9 digits long and are prefixed by GB.
If you are looking for another company's VAT number you should check any invoices that the company has sent to you. However, if there is no such number listed on the invoice you should contact the company immediately so that you can reclaim any VAT they have charged you.
If you are uncertain about the validity of a VAT number you can check it by calling the HMRC VAT helpline. Alternatively you can use the European Union (EU) VAT Information Exchange System (VIES).