Te and temel are both singular. Te is 'thee' (or 'you' in the singular). The difference betwen 'te' and 'temel' here is that the suffix -mel is added, mostly in poetry, to make reference back to 'te'.It's the 'self' of 'thyself'/yourself'.To a Roman both versions, nosce te ipsum' and 'temel nosce' ,would have meant exactly the same.'Ipsum' in 'nosce te ipsum' means 'self' in this context.(It has other uses and meanings but here with 'te' it means ' thyself/yourself'')