If You buy The War Within expansion now (it’s not yet released, but available for pre purchase) You’ll get Dragonflight for free with it: https://us.shop.battle.net/en-us/product/world-of-warcraft-the-war-within
As for how far can You get without buying the expansions - sadly, I don’t know.
Edit: as pointed out below, this link leads to US shop, so for EU accounts You need to use the link provided by Punyelf below.
In WoW (retail)
There was a “level squish” - nowhere in wow can You currently level up to 120. It was possible, but the levels were “squished”, meaning that level 120 characters were converted to level 50 in the new system. This was done in the expansion “Shadowlands” iirc. After that we went to the standard +10 level per expansion, meaning that currently in “Dragonflight” max level is 70.
WoW classic has 3 subsections:
a. Era - just the base game (called vanilla wow), without the expansions - You can level up to 60 there it will stay that way (no expansions will be added there).
b. Classic - currently running the WotLK expansion, with max level of 80. Soon will be changed to Cataclysm expansion, with max level 85.
c. Season of Discovery (SoD) - this servers will only have the base game (vanilla wow without expansions) but there’s a special twist to it: levelling is unlocked in phases, meaning that currently You can level up to 25, soon you will be able to level up to 40, and so on, up to max 60.
If I’m not mistaken, SoD servers will close some time after the last phase hits, with the possibility to transfer the characters to Era servers, but i may be wrong on that account. Don’t quote me on that.
In WoW calssic You choose between the mode (era/classic/sod) while choosing a server iirc.
Good that You saw that, idk how it happend, I believe I was on the correct EU site, just changed language to english before posting as my default is Polish
It’s up to you if you like more the “retail” (Dragonflight / The War Within) version of the game or one of the “classic” versions. You will need to download them separately if you play more than one. The price of classic is included in the subscription you already pay.
The way WoW always worked with subscriptions, is that you pay a subscription which includes all old expansions, but not the newest one. If you want to play the last expansion (highest levels), you need to buy it. Today, as said above, you can buy TWW, and it will include Dragonflight now, just as it will include TWW when released. (Summer-autumn)
If you don’t buy TWW, then when TWW is released, Dragonflight will likely be included automatically in your subscription, meaning you will be able to level to 70 as people do now. You won’t be able to go to the new cap.
Just to add, if you enjoy leveling you can always make a new character, too, to try leveling in other zones and with new abilities. Many players play alts, and possibly a lot of them.
Leveling is relatively a short part of the game now.
In classic (either of the three) it’s more substantial (time consuming).
If you just want to see big numbers, we are enemies some other games might suit you better, BUT in case of WoW, reputation bars sort of fill that purpose. Some currencies as well, goals you work towards - such as a full PvP set for honor. Also, initially, the entire achievement system.
Thing is, in order to allow players to group, leveling has to be finite and furthermore, not too long as most people don’t have enough time. WoW is about a lot of “shorter” progress bars, it’s up to you which one you pick to fill. Chosing them all would take thousands of hours.
The level 25 cap in SoD is basically about that: the realisation that most people have limited time each day, week. (We call that Season of Dadgamers, too.)
1-25 is still 20-35 hours invested.
One more thing. Most people on these forums QQ endlessly about leveling being long. At this point it’s a quick rush to max, then raise gear levels and chase higher difficulties of the same content. Gear-flation aside, most players seem to want GW2-like horizontal progression: buy content, play without gates.