I'm sure you're aware that Xrumer is a very powerful tool, and if used effectively can make thousands and thousands of links beneficial, but at the same time, costly.
There's isn't a specific number that once you've gone past it you've had it. It's all dependant on various different factors including that of the market(s) and keyword(s) you're focusing on.
I'm a great believer in the long term. Meaning, what you want to do is space out your backlinks to look natural - this will either way, if not immediately, be extremely beneficial.
My advice would be to start of slow. Build up some organic traffic through healthy SEO practices such as natural link building and link wheels with various Web 2.0 properties. Familiarise yourself with Xrumer and its functions, test it out on a few domains, and once you're confident you can use it safely - I would recommend starting off with around 500 - 2000 Blasts per day dependent on your competition (If it's high start high, if it's low, start low) .. gradually working your way up to larger numbers.
As a general rule of thumb, I'd never exceed 30,000 Blasts, regardless of whether they stick or not, per week. Making it look naturally, healthy and ethical is absolutely vital - that's why you first must learn how to use it properly.
