I would say it depends. When first built most were for sale. These days many terraced houses are owned by private landlords. Where I live when a house comes up it is bought for rent.
Indeed - cheap housing in those parts of the market is very popular with buy to let landlord investors for a variety of reasons. Terraced housing often meets that landlord market so in some areas substantial numbers (but perhaps not a majority) will be in the private rented sector.
I suspect it is possible if one has the skills to construct a census data query to work out how much terraced housing was in each tenure at the date of the last census.
When first built most were for sale
But when 1st built they would not have been built for sale
for owner occupation as pre mid 20th century housing of this sort aimed at working class people (which majority of terraced homes would have been) would have been owned by private landlords, who would have bought from the developers - often the builder of the original terrace.
This declined after world war one due to the rent controls introduced to stop profiteering during WW1 - which largely existed until the 1988 Housing Act, which set the ground rules for the expansion of Buy to Let from the late 1990s onwards, and has been much questioned in the last few years, including some calling for a return to rent control of some sort.