In competitive matches, it is only 3 subs. In friendlies, you can use the whole squad.

Personally, I never alter my tactics, either during matches, or even when I'm setting the team up for forthcoming matches. In 9 seasons, I've never changed my tackling strength, attacking or defending pressing, marking stratedgy, or anything else. Just set everything to "normal" and left it like that. Hardly ever use arrows either.