Learn German: Wer hat eine Taschenlampe? Ich brauche jetzt eine!

Here, eine acts as a pronoun meaning 'one'. Instead of repeating the noun and saying Ich brauche jetzt eine Taschenlampe, you can just drop the noun to sound more natural. Because Taschenlampe is a feminine noun, the pronoun replacing it is eine.

If it means 'one', why don't we use the number eins?

When you mean 'one of them' (referring to a specific noun), you have to use a pronoun that matches the gender of that noun. You only use eins when counting out loud or when replacing a neuter noun. Since Taschenlampe is feminine, 'one' becomes eine.

What case is the second eine in?

It is in the accusative case, just like the first eine. The verb brauchen (to need) requires a direct object, which always takes the accusative case. For feminine words, the accusative pronoun is eine, which happens to look exactly like the basic nominative form.

Why does jetzt come before eine?

In German, words telling us 'when' something happens (like jetzt, meaning 'now') usually come as early as possible after the verb. Therefore, the time word steps in front of the direct object eine.