I am hoping we have at least 400 delegates going into Tampa. I think less then 300 and we become irrelevant. 300-400 I'm not sure. But over 400 delegates and we might be able to pull this off. I think 300 gives us a voice and 400-500 we might be able to shock the world at a brokered convention
To some extent it's going to be a question of what you mean by "having" delegates. Are these hard pledged delegates? Are the counts including Super Delegates? Are we talking about total delegates after release? The figures shift based on parameters.
There are also the questions of 'how many uncommitted delegates are there' & 'how many delegates to the other candidates have'.
The ratios of those things will have an impact as well, so will other factors like who picked up what in CA, econ, current polling, mood on the ground in Tampa etc.
All my equivocating aside I'd personally love to see 572 or more that we know are ours going into the first ballot (they don't all have to be hard pledged just not hard pledged to
someone else 
). If we're able to command our weight or more of delegate votes going into the first round of balloting that's a very actionable position, especially since regardless of the margin we can expect to be gaining at least
something as delegates become unbound.
So much of this is positioning and organization. Fortunately that's our strong suite and something that these very people will have been getting experience at this year.
There's quite a bit of tension among the various camps (esp at higher levels) and
if Paul positions as the best alternative to their first choice (more likely to be doable among delegates than voters because of the level of investment/personal affinity for their guy) then it could get very interesting indeed.