Basically, the Assault Guards were the urban counterpart to the rural Civil Guards. Both were armed police forces with quasi-military training, and often recruited ex-military members.. Prior to the war, the Civil Guards were used extensively to break up strikes. Sadly, I cannot find any good numbers on either force in Paul Preston's The Spanish Civil War. Also, I did not write down the numbers that I am fairly certain Antony Beevor cited in his book on the war. Online sources vary, sometimes wildly. The Assault Guards in Madrid were critical to suppressing the rebellion at the outbreak of the war, and many of the Civil Guards sided with the Nationalists.

I wish I had better numbers for you, but I hope I was able to explain the difference between the two forces. There are certainly a bewildering array of participants in the Spanish Civil War, and it can take some time to become comfortable with even identifying them. As always, followup questions from OP and others are always encouraged!