"Cia mheud bàta?"
Translation:How many boats?
Because cia mheud takes a singular noun.
Logically, if you are asking the question, you do not know how many there are, so you do not know if there are 1 or 76 - if it should be singular or plural. The convention (and it is only a convention) is that we use the plural in English (even if it turns out there is only 1) and the singular in Gaelic (even if it turns out there are 76).
Historically, there may be deeper reasons that I don't know about.
It's because the words which have gone with the word bàta have caused lenition (the name for when the h is added). Certain words or cases do it, so it's hard to explain why they have done it without you giving specific instances where you've seen it.
Common cases would be with the numbers one and two, both of which cause lenition. Aon bhàta, dà bhàta. Or possibly the dative case (when a noun has a preposition acting upon it). For example, air a' bhàta - on the boat.