One of the design philosophies I like the most about D&D 5E is the idea of Bounded Accuracy. The basic idea is, player characters (PC), non-player characters (NPC) and monsters all fall into a limited range of characteristics. The highest bonus that can be obtained for PC's and NPC's is +5 for attribute scores, this can be higher, but this is and should be rather rare. Monsters can go up to +10, again in theory, they can go higher, but this would be rare. Further, the highest bonus a PC or NPC can get from experience and skill is +6. Additionally, magic bonus's are now limited to +3. In practical terms, this means there is soft limit of +11 and a hard limit of +15 for PC's in game. The video linked below has a very nice discussion about Bounded Accuracy and the affects of rolling with advantage and disadvantage.
To make things easy, I have provided the tables he talks bout below. and can be downloaded here; https://imgur.com/gallery/Y1RA9yq