Learn Spanish (Spain): A lo mejor hay un tesoro en la arena. - Maybe there is a treasure in the sand.

That is the special rule for a lo mejor. Even though it expresses doubt or probability, it always takes the indicative mood. So, we use the indicative hay instead of the subjunctive haya.

Can I use igual instead of a lo mejor in this sentence?

Yes, you can say Igual hay un tesoro en la arena. Both igual and a lo mejor mean 'maybe' or 'perhaps', both take the indicative mood, and both are extremely common in spoken Spanish in Spain.

What does a lo mejor literally mean? It looks like it means 'to the best'.

You are right about the literal translation, but in Spanish it is a fixed expression that simply means 'maybe' or 'perhaps'. It is best to memorize it as a single chunk of vocabulary rather than trying to translate its individual words.

If there were multiple treasures, would hay change to a plural form?

No, the impersonal verb hay (which can mean 'there is' or 'there are') never changes to match a plural noun. To say 'Maybe there are treasures', you would still use the exact same verb form: A lo mejor hay tesoros en la arena.