(...) Cadent a latere tuo mille et decem milia a dextris tuis; ad te autem non appropinquabit. Why is it in the plural and not only "a dextra tua" like "a latere tuo"?
It is a noun, n, pl.: dextra, dextrorum; it means that which is on the right. Like omnia, for example. Omnia mea = all that I have. Dextra mea = that which is on my right.