When asked by TMZ if Victoria has ''signed off'' for a tour, he replied: ''It may be four, not five.''
Simon also admitted he is ''not sure'' whether the 'Wannabe' hitmakers will perform at Prince Harry and Meghan Markle's wedding in May, despite Mel B appearing to confirm they will be earlier this week.
He said: ''I don't know about that, I'm not sure about that one. I'm not sure that will happen.
''Maybe she knows something I don't.''
Mel confirmed she will be attending the royal wedding at St George's Chapel, Windsor Castle, on May 19, and revealed her fellow singers will also be alongside her at the star-studded event.
She said: ''I'm going. I don't know if I should've said that.
''Us five Spice Girls did [get an invitation]. Why am I so honest?''
It comes after the quintet were said to have put pen to paper on contracts for a number of reunion shows after confirming they were back together to work on unspecified projects.
Mel B is said to have signed the contract last month, and documents are said to show specific details for UK and US concerts, including the number and location of shows, and compensation details.
Sources recently claimed the '2 Become 1' hitmakers will perform a ''limited number'' of shows, but will make a huge amount of money from a new range of merchandise.
But Victoria has recently insisted she will not tour with the group.
She said: ''I'm not going on tour. The girls aren't going on tour. It was so great to see the girls...
''There's something so strong in the message of what the Spice Girls stood for. What is that in the future? What does that look like? We were just bouncing ideas around. Brainstorming. But this [fashion] is what I do.''
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