The general practise hereabouts is up to 15 ewes for a ram lamb, preferably none of them first timers. You certainly want him to have some experienced girls, not all first-timers, if he has that sort of number and you want a tight lambing.
And you wouldn't expect a ram lamb to work on the fell, you'd have him and his girls in-bye in a smallish enclosure. You'd probably go in and bring the ewes around him twice a day, too. And if you saw he was devoting himself to one girl only, and ignoring another that was interested in him, you might take the well-covered lady out
And yes, you'd want him to be at least 6 months old, have well-developed testicles, and be at least 60% of his adult weight.
Having said all of which, I've twice used ram lambs on my fleece sheep and some commercials in meandering pastures, 25 acres in total, in 7 fields and pastures, mostly in a long line, and they've both covered everything well. Even the first one, who had two distinct groups who didn't mix (and were often at the diametrically opposed ends of the 25 acres
), managed to get all the girls in both groups.
And on one occasion, BH put a 9 month old Charollais ram lamb in with a large (over 100) batch of ewes early in our season. He left Fred with the big batch for about a week, then took off the marked ewes, and divided the remainder up, leaving Fred with about 30.
People said he'd ruin the boy for future years, but he was fine. He worked well for three years for us, and then had to go because we had too many of his retained daughters.