I have been advised to play 24 or 25 lands in Standard.
The number of basic lands depends on the type of deck you run. So, for example, aggro decks is all about tempo, so it needs few basic lands. As a general guideline this is what it should be like:
Mono-Coloured Aggro Decks
19-21 Basic Lands
Other Mono-Coloured Decks
23-24 Basics
Dual-Coloured Decks
2-3 Fetch Lands
12-14 Basics
3-4 Pain Lands/
Shock Lands (no standard ones currently I think)
Tri-Coloured Decks
4 Fetch Lands
2-4 Pain/
Shock Lands
6-8 Basics
3-4 Scry Lands
4 of a any-colour land (
Mana Confluence)
That's just a general guideline someone told me once. Of course, every deck is different, so it might not help you to dogmatically run 20 Lands for every aggro deck you play. Honestly, tweaking is always needed, and if you're stuck, count the mana symbols of each colour you're playing and work out ratios that way.