Learn Spanish (Latin America): Había tantos grillos que el ruido era terrible. - There were so many crickets that the noise was terrible.

This is the core rule of impersonal haber (meaning 'there is/are' or 'there was/were'). It must always remain singular, even when the object following it—in this case, tantos grillos—is plural. While you will sometimes hear native speakers say habían, standard Spanish grammar requires había.

Why do we use the imperfect tenses (Había and era) instead of the preterite (Hubo and fue)?

Both verbs are setting the scene and describing an ongoing background state in the past. If you used hubo, it would imply a distinct, completed event. Since we are describing what the environment was like over an unspecified period of time, the imperfect is the correct choice.

Why use tantos instead of tan?

Tanto changes to match the gender and number of the noun it modifies, so it becomes tantos before the plural masculine noun grillos. You only use tan (which never changes form) before an adjective or an adverb. For example, if you wanted to say 'The noise was so terrible,' you would use tan: El ruido era tan terrible.

Can I leave out the word que, since we often drop 'that' in English?