Up to three AdSense for content units
Up to three link units
Up to two search boxes

Publishers may not place more than one "large" ad unit per page (300x600 or larger.)

This is for both mobile & desktop.

[support.google.com...]

What Keyplyr said and don't forget Page Level Ads

Also
1. two or more ads may not appear on the screen at the same time on a mobile screen viewpoint.
2. 300x250 or larger units are not allowed above the fold.

1. two or more ads may not appear on the screen at the same time on a mobile screen viewpoint.

Partly because of ad metrics for "viewable" payouts. Conversely, two showing will kill both as an earning point.

(Part of that is humor! Other part is don't space your ads too closely together!)