You get 14 activity every two week period. Not that complicated and that's all you need to know. Table of how many periods for each rank for reference:

1 X 14 = 14
2 X 14 = 28
3 X 14 = 42   --  Jr. Member ( 1.5 month)
4 X 14 = 56
5 X 14 = 70   --  Member (2.5 month)
6 X 14 = 84
7 X 14 = 98
8 X 14 = 112
9 X 14 = 126   --  Full Member (4.5 month)
10 X 14 = 140
11 X 14 = 154
12 X 14 = 168
13 X 14 = 182
14 X 14 = 196
15 X 14 = 210
16 X 14 = 224
17 X 14 = 238
18 X 14 = 252   --  Senior Member (8.5 month)
19 X 14 = 266
20 X 14 = 280
21 X 14 = 294
22 X 14 = 308
23 X 14 = 322
24 X 14 = 336
25 X 14 = 350
26 X 14 = 364
27 X 14 = 378
28 X 14 = 392
29 X 14 = 406
30 X 14 = 420
31 X 14 = 434
32 X 14 = 448
33 X 14 = 462
34 X 14 = 476
35 X 14 = 490   --  Hero Member (1.3 years)

This is good because I see a lot of similar posts in local threads. A lot of newbies are asking about it. However, take note that there's a little delay on the update. I think the update is evey 2nd tuesday, however my status was updated a day before, so just be patient.

Im newbee here, what does it mean by the word necro-ing and necro bump?

Lol. Pretty much what you just did by responding to a post from 2013. A simple google search would have told you everything you needed to know. You should also google 'shitposting'.

I wouldn't call it a penalty.
The more you make posts the more you get activity the more you advance in ranks.
Now the opposite, the more you post in shitty topics the more your posts get deleted the more you lose activity the more you decrease in ranks.
Obviously obvious!

Take a look in theymos post here, it may help you if you want
https://bt.irlbtc.com/view/2385104.0

Lots of good info on how the ranking/activity system work, thanks !

14 activities per 14 days (per period)
For more precision when the periods start https://docs.google.com/spreadsheets/d/12saLhlUoqIdairxzuSPu6EYGrt7FN2lOstO1yDjCEbA/htmlview?pli=1

1- 60 activity to become a member. And since the activity increases by 14 then once someone reach 60 he can advance in activity to 70 (14 activity x 5 periods)

2- you only need to make 1 post per period to have 14 activities. But remember once this post is deleted you will loose your activity for that period so it is wise to make several posts in different topics and subs each period