Contributor link building

Currently doing a contributor link building campaign on pr4 and pr5 websites. How many is safe? The sites are legitimate and I will be donating to charity in exchange for a link.

Should I do only a handful? 10? 20? 100?

Would it look suspicious and get me penalty? I'm sticking to the anchor text rule of mixing it up to look natural.

Many thanks,

if there is mentioned about the exchange google will see it. if not and websites were you are going to get backlinks look like natural and have no doupdful points you can get links.
Anyway try to keep in a natural way and use brand names and partial keywords only. I would do like that

thank you for the reply

Are you using link exchange ? If not, totally 200% safe and secure only. Try to make Brand Keywords (Firm Name). It would be great.

And then, Don't use Google Products (Drive, Hangout and any other). It's safe for your website. Try to create visual content and get some referral backlinks to your main website.

Now, you website totally natural. Safe and secure...