Just to add: you will have the ability to respec, or reassign, your abilities. It comes in the form of an item, the Potion of Clearance, which you can buy for 1,000 crowns from merchants in Novigrad or fairly early on from Keira. If you want to get one immediately, this is where you'll find her:
You'll first encounter her in the Hunting a Witch quest in Velen
There are 69 points from leveling to the level cap of 70 and up to 25 points from Places of Power throughout the world for 94 total available points. There may be further sources for points, but it certainly is not enough to upgrade all skills fully, which amount to 270 total points needed to upgrade all skills:
- Combat (91 to fully upgrade): Fast Attack (20), Strong Attack (20), Defense (13), Marksmanship (18), Battle Trance (20)
- Signs (79 to fully upgrade): Aard (16), Igni (18), Yrden (15), Quen (16), Axii (14)
- Alchemy (90 to fully upgrade): Brewing (18), Oil Preparation (18), Bomb Creation (18), Mutation (18), Toxicity (18)
- General (10 to fully upgrade): 1 point for each; choosing one that complements your armor/playstyle such as Cat School Techniques is often recommended
Given that you won't have the chance to upgrade everything (or even despite it), following down a specialization will probably benefit you the most. The gameplay design behind this is further accentuated by the fact that you only have 12 upgrade slots in which you can actually have the effects of the upgrade active.
Thinking through which skills and skill trees would be most useful to you would be helpful; Combat Fast Attack is a popular choice to "commit" to early on as far as I've seen and, in general, Combat will be universal, followed by Signs and then Alchemy. But if you think differently, you can assign points in different trees or in different orders, since that's the reason that there are so many. There are also many guides online suggesting which upgrades to choose and in which order depending on specific playstyles, if you need some ideas on how other players analytically upgrade. Of course, remember that upgrading in the most ideal manner isn't necessary. While it will make the game's challenges more manageable, there's also the value of discovery in experimenting yourself.
Since it's pricey at the beginning to reassign your points, you might want to upgrade thoughtfully. But if you make or have made some mistakes or if some upgrades are no longer as useful as they were, you'll still have many more points as well as the chance to reset with the Potion of Clearance.