How many pounds, of weight, will a boxer in the middleweight division, normally gain, by fight time, after the official weigh-in?

You're not going to find a cut and dried answer to this. It varies greatly. To boot, see starting with the 2nd paragraph here. : Clearly abuses of it are being noticed.

Rickyj is right. There is not cut and dry answer to this question. I'm a big boxing fan and through many fights that I've watched, fighters gain a few, 3-5 pounds versus others who have ballooned up to 20 pounds after the official weigh-in. Some have what thet call techniques they use to gain some or a lot.
What's your beef, lol, on this?

(Don't forget too. Some boxers will even desire to lose some weight after the weigh-in.)