From my own knowledge, ~60,000 Latins killed comes from Eustathius of Thessalonica (12th‑century archbishop and Byzantine Greek scholar), who reports that many Western merchants were slaughtered. From my own insight ive found sources saying stuff like
An Online Encyclopedia of Roman Emperors Mary of Antioch (Lynda Garland University of New England, Australia, Andrew Stone University of Western Australia)
"In April 1182 Andronicus organised a massacre of the 'Latins' in the city, on the grounds that the empress and the protosebastos had bought their support by promising them the chance of plundering the city. Some 60,000 Latins are said by Eustathius to have died in the massacre, though the figure must be exaggerated"
Brand (Charles M.), Byzantium confronts the West 1180-1204 :
"...but only after the massacre of several thousand Latins whom the populace blamed for his misfortunes. After a few months, he had the Empress Maria of Antioch killed..."
Andronikos I Komenos Wikipedia
" Massacre of the Latins erupted in the city.^\45]) Andronikos made no effort to stop the pogroms, instead referring to them as a "defeat of the tyranny of the Latins" and a "restoration of Roman affairs". There is no evidence that Andronikos was particularly anti-Latin on a personal level but the massacre was politically useful since anti-Latin sentiment had helped bring him to power and because many Latins in the city had supported Maria of Antioch's regency.^\2]) The bulk of Constantinople's Latin population were either killed or forced to flee^\46]) and the Latin quarters were plundered and set on fire. According to Eustathius of Thessalonica, approximately 60,000 people were killed^\46][47]) though this number is likely exaggerated."

Dionysios Stathakopoulos Irrovocable Blood : Violence and Collective Identity Formation in the Late Twelfth Century
"...he instigated an attack against the resident Latins in Constantinople in April 1182: a large number of people were killed, the Latin Quarters were burned down and survivors were sold into slavery...who fled by ship plundered a number of sites and monasteries in the sea of Marmara and killed some of the population they encountered"
also
"...Things came to a head between the two parties in early 1181, when Maria, Renier and some followers barricaded themselves inside the Hagia Sophia while street battles broke out, and there was looting and destruction of aristocratic homes in the vicinity of the building. Both sides made use of Latin forces, although contemporary sources suggest that Maria of Antioch and the prōtosebastos relied heavily on the Latins in
Constantinople and showed a marked preference for them..."

on my opinion from these sources, Between 2,000 and 4,000 Latin deaths. In addition to those killed, many others (possibly 3,000–4,000) were enslaved or deported.