if i buy the battle pass todaay

Yep, if you buy it now you'll get the BP content up to 51. 

https://www.ea.com/en-gb/games/apex-legends/battle-pass

What happens if I buy my Battle Pass late in the season?

You’ll receive all of the rewards up to your Battle Pass level (which is your current level for the season) retroactively! For example, if you’re on level 20 for the season when you buy the Battle Pass, you’ll unlock all rewards for every level up to 20, plus three special Battle Pass skins.

@paganmin333On second thought if i'm not late you can just buy the 800coin battle pass and grind to 77lvl the remaining 4 weeks so you will earn another 800 coins free to buy Season 3 but if you don't buy premium to help you and later reach 100lvl you will miss some rewards especially the lvl 100 Legendary R301 skin, it's awesome!

Plus on lvl 99 you will have 1,200 crafting metals to craft a Legendary.

edit: Don't mind this post Battle pass is 950coins. 

Premium Battle Pass with +25levels required if not enough time to play each week.

@SpyrosFPS The BP is 950, so @paganmin333  would need to hit level 97 to get all their coins back (and 50 extra, cha-ching).

Hitting 97 may be achievable enough, if time allows.

Each week you can get 2 levels for hitting the 10 daily quests, and you can also grind out 5 levels using the escalating XP tracker. That's 9+18+27+36+45 = 135k XP, and after that it's 54k per level per week. 

Add the 3 weekly +1 BP quests, and you can get 10 levels a week relatively easily. 

That brings OP to 91, so the question is whether there are 6 (or more) non-completed weeklies from earlier in the season that can be done to add the extra levels. There may also be some +1 BP quests left to do this week to bring the level up. Grinding out the extra 6 levels (324k XP) may also be an option.