The Archdiocese of Glasgow, by far the country's biggest, has long warned that it will have to make difficult decisions on church numbers as it finds it harder to fill pulpits than pews.
Now new figures show the archdiocese expects to have just 45 priests within two decades, enough for fewer than half its current parishes.
With dozens of priests close to retirement and just two seminarians currently training to replace them, Glasgow Archbishop Philip Tartaglia late last year launched a consultation on parish mergers, promising there would be no "hit list".
However, detailed options being discussed in each of the archdiocese's nine deaneries, or cluster of parishes, suggest dramatic change is on the way.
The South Deanery, in Glasgow's south-west, stands to lose at least seven of its 12 parishes, according to a consultation document. The deanery, which includes Our Lady of Lourdes in Cardonald, stretches from Govan to Pollok and caters for 5715 regular worshippers and 28,000 Catholics.
The document on its future said: "It is conservatively estimated that in 20 years' time there will be 45 priests in the archdiocese, which means roughly five in this deanery."
The paper sets out six options for deploying those five priests. These range from having five parishes, some with more than one church building, to having a single parish with just one building.
The number of diocesan priests in the archdiocese fell from 196 in 1991 -including men whose vocations began during the peak ordinations of the 1950s and 1960s - to just 85 in 2012. The number of parishes in the same period dropped too, but by only 13% from 108 to the current 94.
Speaking in Govan earlier this week, Archbishop Tartaglia said: "The situation and number of our priests is undeniably part of the reason why we are going through this exercise of considering the provision of parishes and priests.
"The other big factor is that communities across the diocese have changed a lot over the decades both in nature and in number."
Archbishop Tartaglia will make all final decisions on mergers and closures but stresses he has tried to strike a clear note of genuine consultation on the issue. "I am not a fan of big planning projects," he said in Govan. "I always have the fear that the local and the particular and the unique will get lost in someone else's big idea. So we must try to be both courageous and sensitive, sometimes a difficult balance to achieve. We need to do what has to be done, but people must not be trampled underfoot.
"As far as possible we need to achieve a consensus between a viable way forward and the realistic hopes of local communities."
The number of priests across Scotland is falling, but the number of Catholics recorded in the last census was up.
Crucially, the Catholic population, like that of the rest of the west of the country, is moving, leaving empty churches in some areas and full and even expanding ones in others.
Congregations are shrinking in council schemes built in the 1950s and 1960s and now being redeveloped.
Those in suburbs, near good Catholic schools, such St Ninian's in Giffnock, are thriving. However, across the archdiocese, numbers are going in the wrong direction for the church. Between 1991 and 2012 there were drops of 41% for mass attendances, 39% for baptisms, 54% for marriages and 14% for funerals.
Church insiders stress the numbers of services of one kind or another still put huge pressure on remaining priests. The slight reduction in funerals still means 85 priests had to conduct a total of 2559 funerals, about 30 each.
Ronnie Convery, spokesman for the Archdiocese of Glasgow, said: "We are involved in a consultation. There is no hit list. No pre-determined list of parishes is facing closure. We are sharing information and projections with people to inform them of the wider picture and to seek their opinions as to the best way forward both for their own area and for the archdiocese at large.
"We are facing three basic issues.
"Glasgow's population has halved in the post-war years while the number of parishes has grown; the number of serving priests is down and, although the overall Catholic population is up, there are fewer regular practising church-goers; and there has been a demo-graphic shift in the Catholic population.
"The once-strong east end has a greatly reduced Catholic population, leaving many parish churches with low numbers, while the south east of the city has seen an influx of Catholics, meaning parish churches are full.
"We need to match our resources to where our people are."
Other denominations have felt similar challenges. The Church of Scotland has merged congregations across Scotland, but tried to keep church buildings open in some rural areas, even if they have to share ministers.
"We all have problems with pulpit supply," said Kirk commentator and former The Herald editor Harry Reid.