Say you go fiend at first with the blade boon. What if you want your character to make a pact with a hexblade patron. Can you have multiple patrons and what's the limit?

You can only have 1 patron at a time. You're selling your soul, or whatever, for the pact. The way I handle this as a DM is I make it so your new patron basically buys out your old contract, like a company selling your debt to a third party. You can make it a lot of fun if you make it like you dont want your contract to be sold and lose your abilities from your old patron, or make your new one a ******, or you two not get along, etc. 

I’ve seen several interesting role plays between different patrons. Most of them have been the same subclass type of patron but there can be exceptions if you choose. Here are just the ones I’ve seen.

Some are just between archdevils, over who takes better care of the warlock.

Different members of an undead council, that each want different things, so they pressure (or bribe) the warlock, to disobey the previous patrons actions and follow them (there were only 4 members on the council for simplicity)

Other more drastic I’ve seen are like, devil (fiend subclass) and angel (celestial domain) bargaining for someone’s soul. 

A character switched from an efreeti patron to an archfey patron they met in the adventure.

Even fjord in critical role kind of had 2 patrons at a time (though not necessarily in a pact)

I guess what I’m saying is, if you want you can be as creative as you can get. With dm permission of course. But definitely have fun with it!

Quote from Jessebruffett >>

The way I handle this as a DM is I make it so your new patron basically buys out your old contract, like a company selling your debt to a third party. 

 This is also a great way to handle a character disobeying their patron. *Asmodeus appears out of nowhere* “Hey you [insert what they did wrong] so I traded you to Buhamut” like in sports when players get traded, warlock patrons can do it too. 

You can multiclass with order of the profane soul blood hunter to get multiple patrons.

I sort of have two patrons right now.  The primary is a succubus.  Given she's not very powerful, she had to seek an outside source for arcane knowledge.  Now she has an agreement with the spirit of an ancient dragon queen who has a vested interest in our party's adventures.  What we're doing is, my first four levels of warlock powers are infernal themed.  Fire, and lots of it, including eldritch blast being changed from force to fire damage (with all the drawback that comes from that).  The dragon was blue, so moving forward, there will be a lot of lightning, instead of fireball, I'll be getting lightning bolt for example.