Yes you can still use your current Futaba Ch. #46 reciever with a new radio providing you stay with Futaba & make sure the new raido is on ch. 46 as well. As for how many channels you will need, ask yourself what kind of flying do you intend to grow into. I rarely see anyone actually "NEED" more than 6 channels. However if you plan to get into really deatailed & scale type Warbirds then you might actually use more than six channels . You basicly need at least 4 to fly, 5 if you want to fly with flaperons, 6 if you use retracts too. I believe if you want true flaps, independent of the ailerons, like most Warbirds use that would require it's own channel too. Then if you wanted to do more things like candy drops, or bombs or what ever you might use yet another channel.
Personally I'm finding my 6 channel JR radio with 10 model memory to be enough for me. But a lot guys like more radio than that. There are also the bells & whistles to be considered. A lot of guys buy more radio than I did for the extra bells & whistles, & not necessarily the extra channels. Which brings it down to a matter of affordablility. If you have the money to spend, then I think most people would say go ahead & get the most radio you can.
Personally I think that is fine for those who can afford it, but the middle of the road radios are very nice too. For instance the Futaba 7 Cap is a very nice radio with a good bit of the bells & whistles while accommodating up to 7 channels. The Futaba 6EXA is also a nice 6 channel radio. So it really boils down to what do you think you will need? how much will you really use? how important are the bells & whistles? & how mouch money do you want to spend on a radio? The other thing is if you are only one receiver deep into Futaba then you may still want to look at JR as well. If I were you I would check out Futaba, JR, & HiTech before you bought anything. One receiver isn't that much. I went into the LHS to buy the Futaba 6 EXA, but when I held the JR6102 in my hand it just felt & looked so much better to me than the Futaba that I had to have it, & that was after I already had 6 planes all on Futaba, but I switched up anyway. I was able to sell & trade every one of my Futaba receivers for JR receivers right here in the RCU marketplace so getting turned around really didn't cost me anything extra. So I would say look online for your best price, but go to your LHS & put those radios you like in your hands to see what feels best for you. Then make your decision as to which brand you like best.
Just to throw a little more confusion into the pot, you might want to look into something with the spectra module as well. Of course it's more money, but it gives you channel versatility. Rather than mis-inform you with incorrect information I would rather let someone with more knowledge than myself explain it to you. In the meantime you can go to Tower Hobbies site & look at radios to inform yourself. I believe if you look at the HiTech Eclpise 7 it has a spectra module option & you can read about it there.
http://www2.towerhobbies.com/cgi-bin...?&I=LXAHK8&P=0
That is the link to the Eclipse 7 with the spectra module. This is for the Tx only, there is no flight pack included at this price, but if you scroll down to tech notes, you can read all about it's features. These HiTech radios are very nice too. They are shift selectable which means you can set them up to go with either Fubtaba or JR receivers. You can work your way back to the rest of the radios to look at from here too. You will not find any of the JR stuff at Tower though. To look at JR radios you will need to go to Horizon Hobbies site.
I hope some of this helps. Good luck.