Can a Thai national, legally have more than one passport?

Yes.

I may not be 100% correct but I think that if you are a legal child of Thai and non-Thai parents you are eligible for a passport from each country. However for example the mother is British then the child at its children are entitled to a British passport through several generations. But if the father is British then the entitlement for a British passport only runs for that child and not for its children.

This only applies in any case if the parent is born of British parents by descent and not by immigration or naturalisation.

This does not apply if the child is illegitimate or adopted as there is no entitlement to a British passport at all.

It is interesting that the mother is permitted to pass on the entitlement and not the father.

It could be a good case for the European Court of Human Rights to deal with.

Can a Thai national, legally have more than one passport?

You want to know what the law says? Read Thailand’s Nationality Act, in English or in Thai. Start reading at Section 7.