Learn Spanish (Latin America): Hay noventa sillas en el restaurante. - There are ninety chairs in the restaurant.

No. Unlike the number one (un / una) or the hundreds (like doscientos / doscientas), the "tens" numbers like noventa (ninety) never change their endings for gender. It is always noventa.

The word for nine is nueve. Why is ninety spelled noventa instead of "nueventa"?

The spelling changes slightly! Just like "five" becomes "fifty" in English, Spanish changes the vowel sound from nueve to make noventa. You will see a similar change with siete (seven) becoming setenta (seventy).

Does hay change to a plural form since there are multiple chairs?

No, hay is a very convenient word because it means both "there is" and "there are". It only has this one form, so you use the exact same word whether there is one chair or ninety.

The word restaurante ends in an "e". How do we know to use el instead of la?