It can work out well with a first time tup but it is a bit of a risk! I had a registered Ryeland tup lamb that I put in with my ewes and he only got about 10 out of 25 in lamb. The following year however, it was 100%.
Worth asking the actual date of birth of tup lambs. Those born in February can probably cope with more than those born in April. I don't recommend using a ram lamb on more than 10, especially if they're ewe lambs or tegs.
I think it would differ between breeds, cant really say that about all ram lambs, maybe its that number for Southdowns? the general known amount around my area is 1:25 for ram lambs however breeds with higher libido can cope with more and breeds with low libido less
Charmoise ram lambs are usually put from 25 or even up to 40 with good results, NZ suffolk ram lambs are recommended for up to 80, with many other breed ram lambs being able to tup around 50 pretty well
However yes I agree that it can be risky relying on just a ram lamb, often people will run a sweeper tup in after a ram lamb has been through a bunch to check if any have come back on heat
I used two ram lambs on their own in own groups last year and they did very well, one got all 30 of his in lamb and the other got all but two (one was a ewe lamb and one an older ewe) in lamb, however yes I took the risk that they may not have worked, but I raddled both and and the ewes were teasered so all were covered in the first two weeks, when I re-raddled the ewes were not re coloured so it was safe(ish) to assume all were tupped, I just had to cross my fingers that they were properly in lamb
This year I am using two rams and two ram lambs, split into two groups, 1 ram going to 30 and the other ram plus ram lambs going to 110