Congress has capped (or limited) the number of available H1B employment visas to 65,000 per year. Of these 65,000, up to 6,800 are set aside for workers from Chile and/or Singapore. The H1B season begins April 1st every year.
Some H1B petitions are exempt from the cap because the worker has an advanced degree. A cap exemption exists to the first 20,000 petitions filed for a beneficiary who holds a U.S. master's degree or higher.
Petitions for new H1B employment at institutions of higher learning or related nonprofit/governmental organizations are also cap-exempt, that is, they don't count towards the 65,000 cutoff.
Finally, petitions filed on behalf of current H1B workers that have already been counted against the cap do not count towards the 65,000.
If USCIS receives more than 65,000 cap-subject petitions on or around April 1st, a visa lottery is conducted. This means that even if you file everything correctly for your foreign-born worker, an H1B visa may be unavailable.