You need "a" before "reliable" to make it grammatical. But if may offer a suggestion, it is a bit long for a motto.
Can you check for me this given below company's motto for trade fairs? It will be placed on the special advertising board. I must be sure that it will be completely correct.
It is going to like be like this:
"20 years of business experience. We are reliable business partner for hundreds of companies."
Is it ok.?
A motto/slogan/tagline should be short(preferably in one line), catchy and does not have to be a full sentence.
How about: Your reliable business partner with 20 years experience.
not a teacher
20 years of experience: a reliable business partner for hundreds of companies.