As the say goes, the more the merrier, however, sometimes this is not so recommendable, since, when you are drafting your arbitration clause, you may increase the cost of your arbitration without noticing it.
The arbitral tribunals typically are composed by one or three arbitrators, but it can go as high as you want in uneven numbers, but the more people assigned to solve your dispute as an arbitrator the costlier your dispute will be, so try not to go bananas with the numbers, but be careful to choose uneven numbers, so you have always a majority or someone with the casting vote.
If you have three arbitrators, the cost will increase, if you have just one arbitrator the cost will be lower, but the decision of having one or three arbitrators, must not be on a pure economic basis, it must be directed at how complex the issue is, to more complexity you need more arbitrators, to a lesser complexity you need a smaller panel.
If you want to know more about the costs of a possible arbitration, almost all the institutional arbitrations have an arbitration calculator, so you can have a rough idea of the cost of the arbitration, which is to cover the expenses by the institution and the expenses and fees by the arbitrators, such will not cover what the law-firms will charge for their representation fees.
As to the language of your arbitration, I know that this can sound a little bit of an oxymoron if you have an arbitration clause within a contract agreed in English and determine that the language of the arbitration to be in Spanish.
Well this is possible, it is because it may happen that the agreement was made in one language, due to the needs of one of the parties, but the actual execution of the contract as a whole was made almost in its entirety in another language, and it is quite normal for this to happen, I was involved in a case that the agreement was in English and actually the arbitration clause was agreed to be the language in English, however, the only time that English was used was during the negotiations and the actual drafting of the contract.
As to the execution of the contract, was everything made under Spanish language, this is because the holdings get into the agreements, but because they tend to have some branches in one place, the whole execution and the day to day operation of the contract was in another language, determining the actual language upon which the parties will execute the contract will help you in saving quite a lot of money in interpreters and translations, so try to choose wisely.
Ultimately if you didn’t choose the language or it happens that in the contracts they were two versions in different languages determining both languages to be the official languages, then will be up to the arbitral tribunal to decide the issue, it will occur that you might have a mix of languages during your arbitration proceedings, so make sure to have people apt to litigate in such languages.