Wikipedia states
During the English Civil War many castles and fortified houses were slighted by the Parliamentarians to stop them being used by the Royalists. Most of the destruction was in Wales, the Midlands, and Yorkshire e.g. Pontefract Castle. The coastal fortifications were spared by the Commonwealth, as they might have been useful for hindering a Royalist or foreign invasion.
I haven't been able to find how many castles and fortified houses were affected.
On 20 October 1645, Parliament resolved that the Committee of Both Kingdoms should consider which castles and fortifications were to be slighted. Through 1646 and beyond, Parliament passed a series of resolutions to slight castles. I'm not sure that a central list exists but you can probably compile one easily if you search for "slighted" in the Journal of the House of Commons. There's a useful overview (but no list) on pages 56-57 of English Civil War Fortifications 1642-51 by Peter Harrington, Donato Spedaliere.
Dr Lila Rakoczy wrote a thesis on the topic. You can read it for free: http://etheses.whiterose.ac.uk/11092/
I think the number of castles slighted was over 100 (can't remember exactly).
