Indefinite Article

The indefinite article is used before singular non-specific nouns. The indefinite article is never used with plural and uncountable nouns.

Use a/an in the following cases:

• The first time you mention or refer to a noun.
• To mean one. 
• To express quantity.

Examples

1. To be able to orchestrate a standalone Veeam backup server, you must deploy an Orchestrator agent on the server. [first mention]
2. Create 2 backup jobs and a replication job. [one]
3. Jobs with a lot / a large amount of metadata may increase the I/O requirements for the configuration database. [quantity]