Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Name | Mineral | Opened | Closed | Years connected | Notes |
| --- | --- | --- | --- | --- | --- |
| Blaen y Cwm | Slate | 1820 | 1911 | 1876–1911 | Connected to the FfR via the Rhiwbach Tramway |
| Bowydd | Slate | 1800 | 1870 | 1854–1870 | Merged with Votty to form Votty & Bowydd quarry |
| Brooke's | Granite | 1919 | ? | ? | Connected by a tramway branching off at the north portal of Moelwyn Tunnel |
| Bwlch-y-Slater | Slate | 1824 | 1960 | 1866–1956 | Connected via the Rhiwbach Tramway |
| Cesail | Slate | 1827 | 1877 | 1836–1877 | Became part of the Oakeley Quarry |
| Conglog | Slate | 1872 | 1920? | 1874–1920 | Remote quarry connected by the Cwmorthin Tramway |
| Cwmorthin | Slate | 1810 | 1937 | 1850–1937 | Connected via the Cwmorthin Tramway; became part of the Oakeley quarry in 1900, the two quarries were connected underground |
| Cwt-y-Bugail | Slate | 1835 | 1972 | 1867–1961 | Last quarry using the (by then partly lifted) Rhiwbach Tramway |
| Diphwys Casson | Slate | 1760 | 1972 | 1860–1955 | Last of the major quarries to be connected to the Ffestiniog Railway |
| Graig Ddu | Slate | 1840 | 1946 |  | Connected via the Festiniog and Blaenau Railway |
| Groby Granite | Granite | 1901 | Circa 1932 | 1914-c.1932 | Connected by a short, steeply graded branch worked by FfR locomotives. |
| Hafodty (Votty) | Slate | 1801 | 1870 | 1851–1870 | Joined with Bowydd quarry to form Votty & Bowydd |
| Llechwedd | Slate | 1846 | Present | 1848-1980s | Currently worked on a small scale and the site of the Llechwedd Slate Caverns tourist attraction |
| Maenofferen | Slate | 1848 | 1999 | 1848–1975 | Became the major owner of quarries connected via inclines at Duffws station, continued to use the stub of the FfR through Blaenau into the mid 1970s. Untopping operations continue in 2007. |
| Nyth-y-Gigfran | Slate | 1840s | 1882 | 1867–1882 | Sometimes known as Glan-y-Pwll quarry. Became part of the Oakeley Quarry |
| Matthews (Rhiwbryfdir) | Slate | 1825 | 1871 | 1842–1871 | Became part of the Oakeley quarry |
| Manod | Slate | c1850 | Present | 1866-1930s | Sometimes known as Bwlch y Slaters and now given the name Cwt y Bugail. Connected to the Festiniog by the Rhiwbach Tramway |
| Moelwyn Slate | Slate | 1826 | c 1900 | 1867–c 1900 | Remote quarry connected to the FfR by a series of seven inclines; worked intermittently and never successfully. |
| Moelwyn Zinc | Zinc, Manganese and Silver | c1892 | by 1919 | 1892-by 1919 | Zinc mine just west of the north portal of the old Moelwyn Tunnel. |
| Oakeley | Slate | 1878 | 2010 | 1878-1950s | Formed by the amalgamation of the Cesail, Matthews and Glan-y-Pwll quarries. Was the largest single quarry in Blaenau Ffestiniog. Was the home of the Gloddfa Ganol tourist attraction in the 1980s and 1990s. Now worked as an open pit by Alfred McAlpine Ltd. |
| Rhiwbach | Slate | 1812 | 1953 | 1860–1953 | Remote quarry situated 4 miles (6.4 km) east of Duffws station and connected via the Rhiwbach Tramway |
| Syenite Setts | Granite | 1874 | 1916 | ? | Quarry later worked as Brooke's |
| Votty & Bowydd | Slate | 1870 | 1964 | 1870–1962 | Formed by the amalgamation of the Hafodty and Bowydd quarries. |
| Wrysgan | Slate | 1830s | 1946 | 1844–1946 | Connected to the railway near Tanygrisiau via a single long incline ending in a tunnel. |
Question:
how many quarries was slate mined at?
Answer:
20