The word "many" here, being part of the OP's question title, is important, and the relative numbers as reported by DavePHD in his answer already addresses this part of the question. As to the whole statement, I would argue that, with "many" being understood as a relative word, the ACLU's statement is "a fair assessment of the historical records."
The core of the ACLU's statement looks to come from One Dies, Get Another: Convict Leasing in the American South.
There are various claims made in the ACLU's statement.
The first claim:
After the Civil War, many states, mostly former slave states,
immediately exploited the 13th Amendment loophole allowing slavery and
involuntary servitude as a punishment for crime.
The convict leasing system was a lasting force in the southern states other than Virginia until the 20th century:
All these factors-declining profitability, the presence of a penitentiary, the development of alternative means of using convict labor, and
significant political pressure against leasing, often connected with a willingness to undertake social expenditures-would be present in varying
degrees in each of the Southern states, permitting public opposition to
the practice, which was never far from the surface, to become effective.
But only in Virginia were these forces in motion from the beginning of
the post-Civil War period. Elsewhere in the South, they would not gather
enough momentum until after the new century had dawned. Virginia's
experience reveals what factors the post-Civil War Southern states would
have to develop before their systems of convict leasing could be overcome. No other state had such a combination of elements (except, in
radically different circumstances, Tennessee) before the twentieth
century.
The next claim:
Many former slaves were arrested and then put back into slave labor conditions through convict leasing
Note that the statement does not specifically make the claim that many former slaves were arrested in order to put them back into slave labor conditions. It simply claims that they were arrested and then put back into slave labor conditions. This is true in the sense that after the civil war the prison population's racial makeup changed rapidly:
(In 1866 Alabama) The convict population had also changed from the previous 99% white to the postwar 90% black.
History of the Alabama Department of Corrections (Thanks to DavePHD for the link from his comment to his answer, and to Aaron for pointing it out)
As to the phrase many former slaves, note that it would not have been at all profitable to have the number of convict laborers rise near to the level of the former slave population:
Slave exploitation has to be looked at in the aggregate because the
slaveowners’ maintenance costs must provide for the subsistence and
reproduction of the entire slave community, including the aged, the
infirm, and children; subsistence means more than simply the cost of
maintaining the individual productive slave. Lessees of convicts, by
contrast, were concerned in the main only with productive laborers.
The sick were refused or, in cases where it was possible, remanded to
jails. In January 1888, at the outset of the TCI lease in Alabama, the
company declined anyone ”who, though able to work, can do no more than
enough to pay for their maintenance,” as the exasperated convict
inspector explained to the governor. Later, in March 1893, the Sloss
Iron and Steel Company, a division of TCI, refused to accept Melissa
Stewart, who was nursing her infant, protesting that ”they had hired
no babies" and dispatching her to the Jefferson County jail.”
The final claim:
(Convict leasing was) a lucrative practice that generated more than 70 percent of total state revenues for the state of Alabama in 1898.
This number was apparently somewhat cherry-picked, given that over the next 8 years the state's budget surplus grew by a factor of 10, reducing the overall percent of state's revenues generated by convict leasing, but the number does appear to be factual:
In 1898 Alabama obtained 73 percent of its total revenue of
$378,120.48 from the hire of its convicts. The state spent $45 on
bedding that year. In 1901-1906 the state's total budget surplus
increased by a factor of ten over that of the previous five-year
period. Of course, the surplus represented more than the hiring of
convicts, but the governor reported that ”about as many [convicts] as
are physically able to be put in the mines are getting out coal by the
ton." Colonies of former convicts had begun to grow up around the
mines. One-third of the free miners at Coalburg were former prisoners
as early as 1889. The convict lease seemed insuperable.
From 1906 to 1910 revenues rose still more, from $1.2 to $1.7 million.
In 1915 receipts of the Convict Department constituted one-sixth of
the state's total income (although some of that revenue came from the
cotton mill at Speigner and from the prison farms, not from leasing
entirely).
