Northamptonshire, England, Baptisms, Marriages and Burials, 1532-1812 contains 4,138,400 records for 343 parishes.
Northamptonshire, England, Marriages, 1754-1912 contains 733,877 records for 325 parishes. Don't forget that from 1837 onward the records are the same as the official marriage certificates. You can save £9.50 for each certificate you might have ordered. Also be sure to check the images for witness names.
In addition there's an updated to a database originally online in February 2014, Northamptonshire, England, Burials, 1813-1912 now containing 484,456 records from 359 parishes.
Northants has borders with eight other counties, counting the short border of the Soke of Peterborough with Lincolnshire, so it may be where ancestors living in adjacent parishes outside the county have their BMBs recorded.
While FamilySearch has no specific database for the county there are 1,011,068 transcript records for Northamptonshire in the England Births and Christenings, 1538-1975 database. There will be entries of interest in England Deaths and Burials, 1538-1991 and England Marriages, 1538–1973 although without the capability of filtering the search to the specific county.
Findmypast has 193,090 transcript records of Northampton marriages and 431,637 for burials.
The Genealogist lists individual Northamptonshire parish records for Castor, Croughton, Dodford, Everdon, Farthingstone, Faxton, Glinton, Harpole, Heyford, Lamport, Maxey, Moulton, Northborough, Peakirk, Rushton, St Peters Northampton, Stoke Bruerne, Stowe Nine Churches, Weston-by-welland And Sutton Bassett