You are kidding, right??
Rarely had more than 3? Well, apart from a brief aberration in 1906-1910, when it dropped to 7 or 8, it's not been in single digits since, well, about 1880, until post-Thatcher days. And for the entire post-war period, up until Thatcher's poll tax, they've had 20 to 35, rising to 50 in the 1931 election.
You also seem to be missing the point. The SNP are effectively saying they'll hold the balance of power, and exert it in favour of 5 million Scots, over the wishes of 58 million non-Scots.
Personally, I think the SNP game is obvious. The objective is independence, and the current tactic (referendum having failed) is to so alienate the rest of us, especially the English, that we want independence from Scotland. It seems to be working admirably, too.
That said, I don't think the SNP are going to have anything like the influence over Labour (assuming a Lab win, but no majority) that they think. If Labour are seen to give in too much, they risk HUGE reputational damage where the bulk of their voters are, and in areas they MUST carry to win power, that could put them out of power for a generation i.e. the south east, east, mand south west.
If they give in too much, they might get power now at the cost of huge, long-term damage for a very long time.