The victims were found in raids at three restaurants in Coventry, Wolverhampton and West Bromwich and an address in Handsworth, Birmingham.
Police suspect the men were working for "next to nothing".
A 49-year-old man and a 45-year-old woman have been arrested on suspicion of slavery offences.
See more stories from Birmingham and the Black Country here
The victims are thought to be mainly Slovakian and Romanian.
The suspects, who are both Slovakian, have been arrested on suspicion of providing staff through a recruitment consultancy.
The raided restaurants were Ming Moon, Stafford Road in Wolverhampton, Wing Wah, Fletchamstead Highway in Coventry and Red Leaf, High Street in West Bromwich.
Five suspected slavery victims were found at the Chinese restaurant in Wolverhampton, 12 at Wing Wah Coventry and three at the an address on Anthrobus Road in Handsworth.
The men will be given the opportunity to speak to specially trained officers and helped into alternative accommodation.
West Midlands Police said signs of modern slavery to look out for include "large groups of people staying in multi-occupancy houses and being transported to and from addresses in vans or minibuses from early in the morning and not coming back until late at night".
Further investigations are underway, the force added.

2 October 2015 Last updated at 18:13 BST
Speaking in an interview with BBC Newsnight's diplomatic and defence editor Mark Urban, he said UK policy in Syria had been hampered by "wishful thinking" - and officials underestimated the staying power of President Bashar al-Assad.
More on this story from Mark Urban on BBC Newsnight at 22:30 BST BBC Two on Friday 2 October, and afterwards on iPlayer

The move comes as the Scottish government continues its investigation of tower blocks in Scotland.
The local government and communities committee said it would review building safety standards "in full, particularly in highly-dense urban areas".
At least 79 people are believed to have died in the Grenfell Tower blaze.
Cladding is thought to have contributed to the rapid spread of the fire.
The Scottish government announced last week that it was working "intensively" with councils to establish the safety of high-rise flats.
On Friday, it said no local authority or housing association high-rise properties in Scotland had the cladding used in Grenfell Tower.
At the weekend, the UK government reported that cladding on 34 tower blocks in 17 council areas in England had failed fire safety tests.
Local government and communities committee convener Bob Doris said: "There are thousands of families and individuals living in high-rise accommodation across Scotland and they are understandably looking for reassurances at this deeply worrying time.
"Whilst both the Scottish government and our social housing providers have moved quickly to take action and to allay fears, it is important that our committee provides an additional layer of scrutiny both in terms of fire safety and building standards.
"That is precisely what we intend to do.
"All parts of the UK must learn from the disaster at Grenfell Tower and we must do all we can to ensure no-one has to experience the loss of a loved one in such tragic circumstances.
"Our committee will be reviewing building safety standards in full, particularly in highly-dense urban areas with many high rises.
"We will speak to local authorities, landlords, tenants and of course the Scottish government as part of this inquiry."
Councils across Scotland have moved to reassure high-rise residents over safety in the wake of the Grenfell fire.
In Aberdeen, the local authority is holding drop-in sessions this week for people living in 20 properties across the city, offering them a chance to raise any concerns.
One tower block resident in West Dunbartonshire said he welcomed news that building standards and conditions would be subject to further scrutiny.
Thomas Sellers, 52, who lives on the eighth floor of a high-rise property in Dumbarton, told BBC Scotland that he and other residents were still looking for reassurance.
He said: "There has been no clarification about whether the building I stay in is safe enough. There are no sprinklers and no main fire alarm for the building and we have no access to the roof as an exit in the event of a fire. We are just looking for answers."
Twenty men have been found living in "poor conditions" during four simultaneous raids across the West Midlands, police said.
[NEXT_CONCEPT]
Russian's intervention in Syria is "hugely significant" says the UK's former senior military adviser in the Middle East Lt Gen Sir Simon Mayall.
[NEXT_CONCEPT]
The safety of Scotland's high-rise flats is to be examined by a Holyrood committee following the Grenfell Tower tragedy in London.