I delete numbers all the time - I don't hoard them or keep a phone full of hundreds of flakey #'s in the fashion that some guys do.
Have to keep in mind that the overwhelming majority of numbers you get these days, online or off, are simply weak leads. If a girl really liked you, she'd be proactively opening you on a site, or asking you when you were free, or be back at your apartment already. That level of interest happens to me from time to time but it's rare.
Girls who give you their number in person and say "Ok, text me sometime", who haven't already let you escalate pretty close to sex, are for the most part marginal from the get-go. Girls who you have to grind a number out of on an app that didn't open you first are also marginal; if I had to open her first and chat for 10 messages for her to give up the digits, at the very least I want to see that she's getting back promptly via text for the interaction to continue.
Roosh made a post about warning signs to look for that indicate a bang is probably not on the table:
http://www.rooshv.com/5-warning-signs-to...first-date
For my part I certainly agree. I've never banged a girl who waited more than 3 hours to reply, or flaked and tried to reschedule, or when I got the impression that I was chasing real hard while she was just kicking back doing her thing. Over time, I got an intuition on which girls were investing in me and which girls were simply passing the time.
Unless she's absolutely a stunner, ejecting early at the first sign of ambivalence on her part has usually been a pretty solid option. IMO a girl who never replied once isn't even worthy of a "restart" - there's nothing to restart. Girls who are jumping your bones in your car after a good first date but then get flakey afterwards might get a restart. Not some rando you ain't never even got out.