Twitter is down to fewer than 550 full-time engineers

Twitter's full-time headcount has dwindled to approximately 1,300 active, working employees, including fewer than 550 full-time engineers by title, according to internal records viewed by CNBC. Around 75 of the company's 1,300 employees are on leave including about 40 engineers.

The company's trust and safety team, which makes policy recommendations, design and product changes with the aim of keeping all of Twitter's users safe, is down to fewer than 20 full-time employees.

Internal records indicate that there are also about 1,400 non-working employees of Twitter who are still being paid, but are no longer expected to fulfill their old responsibilities at the social media company. Many of them resigned when CEO Elon Musk sent out a "pledge" asking them to commit to "hardcore" work at Twitter 2.0 including long hours. 

Musk has contradicted the internal records obtained by CNBC in a series of tweets, and claimed that Twitter now has about 2,300 full-time working employees and thousands of contractors. CNBC contacted Twitter for clarification and comment but did not immediately hear back.

Under Musk's management, Twitter has slashed headcount through mass layoffs, other terminations and changes that compelled many to resign, including the end of a work-from-home forever policy that had been put in place under former CEO Jack Dorsey.

Before Musk led a $44 billion leveraged buyout of Twitter last year, Twitter's headcount stood at about 7,500 employees. Layoffs were rumored internally and expected to take place whether Musk's takeover went through or not. However, Musk has cut Twitter personnel far more than many expected — or by about 80% according to the internal records and two recent employees who spoke with CNBC.

According to an engineer who resigned from the company, the loss of employees and reduced headcount will make it harder to maintain the service reliably while still building new features.

This person, who asked to remain unnamed while discussing a prior employer, explained that the company's code base is massive, and it requires knowledge of different platforms and programming languages to maintain different parts of Twitter — advertising services versus the core timeline, for instance. Engineers' skill sets, they said, are not necessarily transferrable across all of these. And it will be hard to train engineers after losing so much institutional knowledge, this person added.

In addition to around 1,300 full-time working employees, Musk also has authorized about 130 people from his other businesses, including Tesla, SpaceX and The Boring Company, as well as talent from venture funds and other firms, to work at Twitter.

Since taking over Twitter, Musk has faced a shareholder backlash at Tesla for being distracted, for stirring up political controversy with his strategy at Twitter, and for selling billions of dollars worth of his Tesla shares to finance his Twitter takeover.

Clarification: This story has been updated to note that the 1,300 people are active, working employees at Twitter. Another 1,400 people are non-working, but paid employees of Twitter.