and no of course you can't wear multiple compaigns otherwise all peopl would enroll and be rich
Most campaigns do have an upper limit tough... It's usually maximum 100 posts/week or something (really depends on the campaign)... You can post more, but won't be payed for the "extra" posts. Also, some campaigns do not pay for posts in off-topic, altcoins,...
A reasonably complete overview:
https://bitcointalksearch.org/topic/overview-of-bitcointalk-signature-ad-campaigns-last-update-01-jan-23-615953
or which is the best one do u guys think ?? i mean the highest paying.
The one in my sig
No, it depends on your posting habits... Count how much posts you make on average, and on which subforum you post most of the time... Then calculate how much each sig campaign would pay you with your posting habit...
Also, i try to avoid sigcampaigns that are obvious scams... but that's up to you